# Analyzing Data with GraphPad Prism

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2 Analyzing one group Frequency distributions A frequency distribution shows the distribution of Y values in each data set. The range of Y values is divided into bins and Prism determines how many values fall into each bin. Click Analyze and select built-in analyses. Then choose Frequency Distribution from the list of statistical analyses to bring up the parameters dialog. Entering data to analyze one group Prism can calculate descriptive statistics of a column of values, test whether the group mean or median differs significantly from a hypothetical value, and test whether the distribution of values differs significantly from a Gaussian distribution. Prism can also create a frequency distribution histogram. When you choose a format for the data table, choose a single column of Y values. Choose to have no X column (or choose an X column for labeling only). Enter the values for each group into a separate Y column. If your data table includes an X column, be sure to use it only for labels. Values you enter into the X column will not be analyzed by the analyses discussed here. If you format the Y columns for entry of replicate values (for example, triplicates), Prism first averages the replicates in each row. It then calculates the column statistics on the means, without considering the SD or SEM of each row. If you enter ten rows of triplicate data, the column statistics are calculated from the ten row means, not from the 30 individual values. If you format the Y columns for entry of mean and SD (or SEM) values, Prism calculates column statistics for the means and ignores the SD or SEM values you entered. Prism provides two analyses for analyzing columns of numbers. Use the frequency distribution analysis to tabulate the distribution of values in the column (and create a histogram). Use the column statistics analysis to compute descriptive statistics, to test for normality, and to compare the mean (or median) of the group against a theoretical value using a one-sample t test or the Wilcoxon rank sum test. Histograms generally look best when the bin width is a round number and there are bins. Either use the default settings, ot clear the auto option to enter the center of the first bin and the width of all bins. If you entered replicate values, Prism can either place each replicate into its appropriate bin, or average the replicates and only place the mean into a bin. All values too small to fit in the first bin are omitted from the analysis. Enter an upper limit to omit larger values from the analysis. Select Relative frequencies to determine the fraction of values in each bin, rather than the number of values in each bin. For example, if you have 50 data points of which 15 fall into the third bin, the results for the third bin will be 0.30 (15/50) rather than 15. Select Cumulative binning to see the cumulative distribution. Each bin contains the number of values that fall within or below that bin. By definition, the last bin contains the total number of values. Analyzing one group 23 Analyzing Data with GraphPad Prism 24 Copyright (c) 1999 GraphPad Software Inc.

3 Column statistics From your data table, click Analyze and choose built-in analyses. Then choose Column Statistics from the list of statistical analyses to bring up the parameters dialog. To interpret the results, see "Interpreting descriptive statistics" on page 26,"The results of normality tests" on page 29, "The results of a one-sample t test" on page 30, or "The results of a Wilcoxon rank sum test" on page 34. Interpreting descriptive statistics SD and CV The standard deviation (SD) quantifies variability. If the data follow a bellshaped Gaussian distribution, then 68% of the values lie within one SD of the mean (on either side) and 95% of the values lie within two SD of the mean. The SD is expressed in the same units as your data. Prism calculates the SD using the equation below. (Each y i is a value, ymean is the average, and N is sample size). SD = ( yi ymean) 2 N 1 Choose the descriptive statistics you want to determine. Note that CI means confidence interval. Check "test whether the distribution is Gaussian" to perform a normality test. To determine whether the mean of each column differs from some hypothetical value, choose the one-sample t test, and enter the hypothetical value, which must come from outside this experiment. The hypothetical value could be a value that indicates no change such as 0.0, 1.0 or 100, or it could come from theory or previous experiments. The one-sample t test assumes that the data are sampled from a Gaussian distribution. The Wilcoxon rank sum test compares whether the median of each column differs from a hypothetical value. It does not assume that the distribution is Gaussian, but generally has less power than the one-sample t test. Choosing between a one-sample t test and a Wilcoxon rank sum test involves similar considerations to choosing between an unpaired t test and the Mann-Whitney nonparametric test. See "t test or nonparametric test?" on page 42. The standard deviation computed this way (with a denominator of N-1) is called the sample SD, in contrast to the population SD which would have a denominator of N. Why is the denominator N-1 rather than N? In the numerator, you compute the difference between each value and the mean of those values. You don t know the true mean of the population; all you know is the mean of your sample. Except for the rare cases where the sample mean happens to equal the population mean, the data will be closer to the sample mean than it will be to the population mean. This means that the numerator will be too small. So the denominator is reduced as well. It is reduced to N-1 because that is the number of degrees of freedom in your data. There are N-1 degrees of freedom, because you could calculate the remaining value from N-1 of the values and the sample mean. The coefficient of variation (CV) equals the standard deviation divided by the mean (expressed as a percent). Because it is a unitless ratio, you can compare the CV of variables expressed in different units. It only makes sense to report CV for variables where zero really means zero, such as mass or enzyme activity. Don't calculate CV for variables, such as temperature, where the definition of zero is somewhat arbitrary. SEM The standard error of the mean (SEM) is a measure of how far your sample mean is likely to be from the true population mean. The SEM is calculated by this equation: Analyzing one group 25 Analyzing Data with GraphPad Prism 26 Copyright (c) 1999 GraphPad Software Inc.

4 SEM = SD N With large samples, the SEM is always small. By itself, the SEM is difficult to interpret. It is easier to interpret the 95% confidence interval, which is calculated from the SEM. 95% Confidence interval The confidence interval quantifies the precision of the mean. The mean you calculate from your sample of data points depends on which values you happened to sample. Therefore, the mean you calculate is unlikely to equal the overall population mean exactly. The size of the likely discrepancy depends on the variability of the values (expressed as the SD) and the sample size. Combine those together to calculate a 95% confidence interval (95% CI), which is a range of values. You can be 95% sure that this interval contains the true population mean. More precisely, if you generate many 95% CIs from many data sets, you expect the CI to include the true population mean in 95% of the cases and not to include the true mean value in the other 5% of the cases. Since you don't know the population mean, you'll never know when this happens. The confidence interval extends in each direction by a distance calculated from the standard error of the mean multiplied by a critical value from the t distribution. This value depends on the degree of confidence you want (traditionally 95%, but it is possible to calculate intervals for any degree of confidence) and on the number of degrees of freedom in this experiment (N-1). With large samples, this multiplier equals With smaller samples, the multiplier is larger. Quartiles and range Quartiles divide the data into four groups, each containing an equal number of values. Quartiles divided by the 25 th percentile, 50 th percentile, and 75 th percentile. One quarter of the values are less than or equal to the 25 th percentile. Three quarters of the values are less than or equal to the 75 th percentile. The median is the 50 th percentile. Prism computes percentile values by first computing R= P*(N+1)/100, where P is 25, 50 or 75 and N is the number of values in the data set. The result is the rank that corresponds to the percentile value. If there are 68 values, the 25 th percentile corresponds to a rank equal to.25*69= So the 25 th percentile lies between the value of the 17 th and 18 th value (when ranked from low to high). Prism computes the 25 th percentile as the average of those two values (some programs report the value one quarter of the way between the two). Analyzing one group 27 Geometric mean The geometric mean is the antilog of the mean of the logarithm of the values. It is less affected by outliers than the mean. Which error bar should you choose? It is easy to be confused about the difference between the standard deviation (SD) and standard error of the mean (SEM). The SD quantifies scatter how much the values vary from one another. The SEM quantifies how accurately you know the true mean of the population. The SEM gets smaller as your samples get larger. This makes sense, because the mean of a large sample is likely to be closer to the true population mean than is the mean of a small sample. The SD does not change predictably as you acquire more data. The SD quantifies the scatter of the data, and increasing the size of the sample does not increase the scatter. The SD might go up or it might go down. You can't predict. On average, the SD will stay the same as sample size gets larger. If the scatter is caused by biological variability, your probably will want to show the variation. In this case, graph the SD rather than the SEM. You could also instruct Prism to graph the range, with error bars extending from the smallest to largest value. Also consider graphing every value, rather than using error bars. If you are using an in vitro system with no biological variability, the scatter can only result from experimental imprecision. In this case, you may not want to show the scatter, but instead show how well you have assessed the mean. Graph the mean and SEM or the mean with 95% confidence intervals. Ideally, the choice of which error bar to show depends on the source of the variability and the point of the experiment. In fact, many scientists always show the mean and SEM, to make the error bars as small as possible. Other results For other results, see "The results of normality tests" on page 29, "The results of a one-sample t test" on page 30, or "The results of a Wilcoxon rank sum test" on page 34. The results of normality tests How the normality test works Prism tests for deviations from Gaussian distribution using the Kolmogorov- Smirnov (KS) test. Since the Gaussian distribution is also called the Normal Analyzing Data with GraphPad Prism 28 Copyright (c) 1999 GraphPad Software Inc.

9 Row means and totals If you enter data with replicate Y values, Prism will automatically graph mean and SD (or SEM). You don t have to choose any analyses. Use settings on the Symbols dialog (double-click on any symbol to see it) to plot individual points or to choose SD, SEM, 95%CI or range error bars. To view a table of mean and SD (or SEM) values, use the Row Means or Totals analysis. Click Analyze and choose built-in analyses. Then choose Row means and totals from the list of data manipulations to bring up this dialog. First, decide on the scope of the calculations. If you have entered more than one data set in the table, you have two choices. Most often, you ll calculate a row total/mean for each data set. The results table will have the same number of data sets as the input table. The other choice is to calculate one row total/mean for the entire table. The results table will then have a single data set containing the grand totals or means. Then decide what to calculate: row totals, row means with SD or row means with SEM. To review the difference between SD and SEM see "Interpreting descriptive statistics" on page 26. Analyzing one group 37

12 You are sure that the population is far from Gaussian. Before choosing a nonparametric test, consider transforming the data (i.e. to logarithms, reciprocals). Sometimes a simple transformation will convert non-gaussian data to a Gaussian distribution. See Advantages of transforming the data first on page 40. In many situations, perhaps most, you will find it difficult to decide whether to select nonparametric tests. Remember that the Gaussian assumption is about the distribution of the overall population of values, not just the sample you have obtained in this particular experiment. Look at the scatter of data from previous experiments that measured the same variable. Also consider the source of the scatter. When variability is due to the sum of numerous independent sources, with no one source dominating, you expect a Gaussian distribution. Prism performs normality testing in an attempt to determine whether data were sampled from a Gaussian distribution, but normality testing is less useful than you might hope (see "The results of normality" on page 29). Normality testing doesn t help if you have fewer than a few dozen (or so) values. Your decision to choose a parametric or nonparametric test matters the most when samples are small for reasons summarized here: Normality test Assume equal variances? Useful. Use a normality test to determine whether the data are sampled from a Gaussian population. Not very useful. Little power to discriminate between Gaussian and non-gaussian populations. Small samples simply don't contain enough information to let you make inferences about the shape of the distribution in the entire population. The unpaired t test assumes that the two populations have the same variances. Since the variance equals the standard deviation squared, this means that the populations have the same standard deviation). A modification of the t test (developed by Welch) can be used when you are unwilling to make that assumption. Check the box for Welch's correction if you want this test. This choice is only available for the unpaired t test. With Welch's t test, the degrees of freedom are calculated from a complicated equation and the number is not obviously related to sample size. Large samples (> 100 or so) Small samples (<12 or so) Welch's t test is used rarely. Don't select it without good reason. Parametric tests Nonparametric test Robust. P value will be nearly correct even if population is fairly far from Gaussian. Powerful. If the population is Gaussian, the P value will be nearly identical to the P value you would have obtained from a parametric test. With large sample sizes, nonparametric tests are almost as powerful as parametric tests. Not robust. If the population is not Gaussian, the P value may be misleading. Not powerful. If the population is Gaussian, the P value will be higher than the P value obtained from a t test. With very small samples, it may be impossible for the P value to ever be less than 0.05, no matter how the values differ. One- or two-tail P value? If you are comparing two groups, you need to decide whether you want Prism to calculate a one-tail or two-tail P value. To understand the difference, see One- vs. two-tail P values on page 10. You should only choose a one-tail P value when: You predicted which group would have the larger mean (if the means are in fact different) before collecting any data. You will attribute a difference in the wrong direction (the other group ends up with the larger mean), to chance, no matter how large the difference. Since those conditions are rarely met, two-tail P values are usually more appropriate. Confirm test selection Based on your choices, Prism will show you the name of the test you selected. t tests and nonparametric comparisons 43 Analyzing Data with GraphPad Prism 44 Copyright (c) 1999 GraphPad Software Inc.

23 data to create a Gaussian distribution, choose a nonparametric test. See "ANOVA or nonparametric test?" on page 68. Choosing one-way ANOVA and related analyses Start from the data or results table you wish to analyze (see "Entering data for ANOVA (and nonparametric tests)" on page 65. Press the Analyze button and choose built-in analyses. Then select One-way ANOVA from the list of statistical analyses. If you don't wish to analyze all columns in the table, select the columns you wish to compare. Press ok to bring up the Parameters dialog. Repeated measures test? You should choose repeated measures test when the experiment uses matched subjects. Here are some examples: You measure a variable in each subject before, during and after an intervention. You recruit subjects as matched sets. Each subject in the set has the same age, diagnosis and other relevant variables. One of the set gets treatment A, another gets treatment B, another gets treatment C, etc. You run a laboratory experiment several times, each time with a control and several treated preparations handled in parallel. You measure a variable in triplets, or grandparent/parent/child groups. More generally, you should select a repeated measures test whenever you expect a value in one group to be closer to a particular value in the other groups than to a randomly selected value in the another group. Ideally, the decision about repeated measures analyses should be made before the data are collected. Certainly the matching should not be based on the variable you are comparing. If you are comparing blood pressures in two groups, it is ok to match based on age or zip code, but it is not ok to match based on blood pressure. The term repeated measures applies strictly when you give treatments repeatedly to one subject. The other examples are called randomized block experiments (each set of subjects is called a block and you randomly assign treatments within each block). The analyses are identical for repeated measures and randomized block experiments, and Prism always uses the term repeated measures. ANOVA or nonparametric test? ANOVA, as well as other statistical tests, assumes that you have sampled data from populations that follow a Gaussian bell-shaped distribution. Biological data never follow a Gaussian distribution precisely, because a Gaussian distribution extends infinitely in both directions, so it includes both infinitely low negative numbers and infinitely high positive numbers! But many kinds of biological data follow a bell-shaped distribution that is approximately Gaussian. Because ANOVA works well even if the distribution is only approximately Gaussian (especially with large samples), these tests are used routinely in many fields of science. An alternative approach does not assume that data follow a Gaussian distribution. In this approach, values are ranked from low to high and the analyses are based on the distribution of ranks. These tests, called nonparametric tests, are appealing because they make fewer assumptions about the distribution of the data. But there is a drawback. Nonparametric tests are less powerful than the parametric tests that assume Gaussian distributions. This means that P values tend to be higher, making it harder to detect real differences as being statistically significant. If the samples are large the difference in power is minor. With small samples, nonparametric tests have little power to detect differences. With very small groups, nonparametric tests have zero power the P value will always be greater than You may find it difficult to decide when to select nonparametric tests. You should definitely choose a nonparametric test in these situations: The outcome variable is a rank or score with only a few categories. Clearly the population is far from Gaussian in these cases. One-way ANOVA and nonparametric comparisons 65 Analyzing Data with GraphPad Prism 66 Copyright (c) 1999 GraphPad Software Inc.

24 One, or a few, values are off scale, too high or too low to measure. Even if the population is Gaussian, it is impossible to analyze these data with a t test or ANOVA. Using a nonparametric test with these data is easy. Assign an arbitrary low value to values too low to measure, and an arbitrary high value to values too high to measure. Since the nonparametric tests only consider the relative ranks of the values, it won t matter that you didn t know one (or a few) of the values exactly. You are sure that the population is far from Gaussian. Before choosing a nonparametric test, consider transforming the data (perhaps to logarithms or reciprocals). Sometimes a simple transformation will convert non-gaussian data to a Gaussian distribution. In many situations, perhaps most, you will find it difficult to decide whether to select nonparametric tests. Remember that the Gaussian assumption is about the distribution of the overall population of values, not just the sample you have obtained in this particular experiment. Look at the scatter of data from previous experiments that measured the same variable. Also consider the source of the scatter. When variability is due to the sum of numerous independent sources, with no one source dominating, you expect a Gaussian distribution. Prism performs normality testing in an attempt to determine whether data were sampled from a Gaussian distribution, but normality testing is less useful than you might hope (see "The results of normality tests" on page 29). Normality testing doesn t help if you have fewer than a few dozen (or so) values. Your decision to choose a parametric or nonparametric test matters the most when samples are small for reasons summarized here: Parametric tests Nonparametric test Normality test Large samples (> 100 or so) Robust. P value will be nearly correct even if population is fairly far from Gaussian. Powerful. If the population is Gaussian, the P value will be nearly identical to the P value you would have obtained from a parametric test. With large sample sizes, nonparametric tests are almost as powerful as parametric tests. Useful. Use a normality test to determine whether the data are sampled from a Gaussian population. Small samples (<12 or so) Not robust. If the population is not Gaussian, the P value may be misleading. Not powerful. If the population is Gaussian, the P value will be higher than the P value obtained from ANOVA. With very small samples, it may be impossible for the P value to ever be less than 0.05, no matter how the values differ. Not very useful. Little power to discriminate between Gaussian and non-gaussian populations. Small samples simply don't contain enough information to let you make inferences about the shape of the distribution in the entire population. Which post test? If you are comparing three or more groups, you may pick a post test to compare pairs of group means. Prism offers these choices: Choosing an appropriate post test is not straightforward, and different statistics texts make different recommendations. Select Dunnett s test if one column represents control data, and you wish to compare all other columns to that control column but not to each other. One-way ANOVA and nonparametric comparisons 67 Analyzing Data with GraphPad Prism 68 Copyright (c) 1999 GraphPad Software Inc.

27 comparisons, not for each individual comparison. If the null hypothesis is true (all the values are sampled from populations with the same mean), then there is only a 5% chance that any one or more comparisons will have a P value less than Prism also reports a 95% confidence interval for the difference between each pair of means (except for the Newman-Keuls post test, which cannot be used for confidence intervals). These intervals account for multiple comparisons. There is a 95% chance that all of these intervals contain the true differences between population, and only a 5% chance that any one or more of these intervals misses the true difference. A 95% confidence interval is computed for the difference between each pair of means, but the 95% probability applies to the entire family of comparisons, not to each individual comparison. How to think about results from one-way ANOVA One-way ANOVA compares the means of three or more groups, assuming that data are sampled from Gaussian populations. The most important results are the P value and the post tests. The overall P value answers this question: If the populations really have the same mean, what is the chance that random sampling would result in means as far apart from one another (or more so) as you observed in this experiment? If the overall P value is large, the data do not give you any reason to conclude that the means differ. Even if the true means were equal, you would not be surprised to find means this far apart just by coincidence. This is not the same as saying that the true means are the same. You just don t have compellilng evidence that they differ. If the overall P value is small, then it is unlikely that the differences you observed are due to a coincidence of random sampling. You can reject the idea that all the populations have identical means. This doesn t mean that every mean differs from every other mean, only that at least one differs from the rest. Look at the results of post tests to identify where the differences are. Statistically significant is not the same as scientifically important. Before interpreting the P value or confidence interval, you should think about the size of the difference you are looking for. How large a difference would you consider to be scientifically important? How small a difference would you consider to be scientifically trivial? Use scientific judgment and common sense to answer these questions. Statistical calculations cannot help, as the answers depend on the context of the experiment. As discussed below, you will interpret the post test results differently depending on whether the difference is statistically significant or not. If the difference is statistically significant If the P value for a post test is small, then it is unlikely that the difference you observed is due to a coincidence of random sampling. You can reject the idea that those two populations have identical means. Because of random variation, the difference between the group means in this experiment is unlikely to equal the true difference between population means. There is no way to know what that true difference is. With most post tests (but not the Newman-Keuls test), Prism presents the uncertainty as a 95% confidence interval for the difference between all (or selected) pairs of means. You can be 95% sure that this interval contains the true difference between the two means. To interpret the results in a scientific context, look at both ends of the confidence interval and ask whether they represent a difference between means that would be scientifically important or scientifically trivial. How to think about the results of post tests If the columns are organized in a natural order, the post test for linear trend tells you whether the column means have a systematic trend, increasing (or decreasing) as you go from left to right in the data table. See "Post test for a linear trend" on page 74. With other post tests, look at which differences between column means are statistically significant. For each pair of means, Prism reports whether the P value is less than 0.05, 0.01 or One-way ANOVA and nonparametric comparisons 73 Analyzing Data with GraphPad Prism 74 Copyright (c) 1999 GraphPad Software Inc.

30 Is the factor fixed rather than random? Prism performs Type I ANOVA, also known as fixed-effect ANOVA. This tests for differences among the means of the particular groups you have collected data from. Type II ANOVA, also known as random-effect ANOVA, assumes that you have randomly selected groups from an infinite (or at least large) number of possible groups, and that you want to reach conclusions about differences among ALL the groups, even the ones you didn t include in this experiment. Type II random-effects ANOVA is rarely used, and Prism does not perform it. If you need to perform ANOVA with random effects variables, consider using the program NCSS from The results of repeated measures one-way ANOVA How repeated measures ANOVA works Repeated measures one-way ANOVA compares three or more matched groups, based on the assumption that the differences between matched values are Gaussian. For example, one-way ANOVA may compare measurements made before, during and after an intervention, when each subject was assessed three times.the P value answers this question: If the populations really have the same mean, what is the chance that random sampling would result in means as far apart (or more so) as observed in this experiment? ANOVA table The P value is calculated from the ANOVA table. With repeated measures ANOVA, there are three sources of variability: between columns (treatments), between rows (individuals) and random (residual). The ANOVA table partitions the total sum-of-squares into those three components. It then adjusts for the number of groups and number of subjects (expressed as degrees of freedom) to compute two F ratios. The main F ratio tests the null hypothesis that the column means are identical. The other F ratio tests the null hypothesis that the row means are identical (this is the test for effective matching). In each case, the F ratio is expected to be near 1.0 if the null hypothesis is true. If F is large, the P value will be small. The circularity assumption Repeated measures ANOVA assumes that the random error truly is random. A random factor that causes a measurement in one subject to be a bit high (or low) should have no affect on the next measurement in the same subject. This assumption is called circularity or sphericity. It is closely related to another term you may encounter, compound symmetry. Repeated measures ANOVA is quite sensitive to violations of the assumption of circularity. If the assumption is violated, the P value will be too low. You ll violate this assumption when the repeated measurements are made too close together so that random factors that cause a particular value to be high (or low) don t wash away or dissipate before the next measurement. To avoid violating the assumption, wait long enough between treatments so the subject is essentially the same as before the treatment. When possible, also randomize the order of treatments. You only have to worry about the assumption of circularity when you perform a repeated measures experiment, where each row of data represents repeated measurements from a single subject. It is impossible to violate the assumption with randomized block experiments, where each row of data represents data from a matched set of subjects. See "Repeated measures test?" on page 67 Was the matching effective? A repeated measures experimental design can be very powerful, as it controls for factors that cause variability between subjects. If the matching is effective, the repeated measures test will yield a smaller P value than an ordinary ANOVA. The repeated measures test is more powerful because it separates between-subject variability from within-subject variability. If the pairing is ineffective, however, the repeated measures test can be less powerful because it has fewer degrees of freedom. Prism tests whether the matching was effective and reports a P value that tests the null hypothesis that the population row means are all equal. If this P value is low, you can conclude that the matching is effective. If the P value is high, you can conclude that the matching was not effective and should consider using ordinary ANOVA rather than repeated measures ANOVA. How to think about results from repeated measures oneway ANOVA Repeated measures ANOVA compares the means of three or more matched groups. The term repeated measures strictly applies only when you give treatments repeatedly to each subject, and the term randomized block is used when you randomly assign treatments within each group (block) of matched subjects. The analyses are identical for repeated measures and randomized block experiments, and Prism always uses the term repeated measures. One-way ANOVA and nonparametric comparisons 79 Analyzing Data with GraphPad Prism 80 Copyright (c) 1999 GraphPad Software Inc.

35 Two-way analysis of variance Introduction to two-way ANOVA Two-way ANOVA, also called two factor ANOVA, determines how a response is affected by two factors. For example, you might measure a response to three different drugs in both men and women. Two-way ANOVA simultaneously asks three questions: 1. Does the first factor systematically affect the results? In our example: Are the mean responses the same for all three drugs? 2. Does the second factor systematically affect the results? In our example: Are the mean responses the same for men and women? 3. Do the two factors interact? In our example: Are the difference between drugs the same for men and women? Or equivalently, is the difference between men and women the same for all drugs? Although the outcome measure (dependent variable) is a continuous variable, each factor must be categorical, for example: male or female; low, medium or high dose; wild type or mutant. ANOVA is not an appropriate test for assessing the effects of a continuous variable, such as blood pressure or hormone level (use a regression technique instead). Prism can perform ordinary two-way ANOVA accounting for repeated measures when there is matching on one of the factors (but not both). Prism cannot perform any kind of nonparametric two-way ANOVA. The ANOVA calculations ignore any X values. You may wish to format the X column as text in order to label your rows. Or you may omit the X column altogether, or enter numbers. You may leave some replicates blank, and still perform ordinary two-way ANOVA (so long as you enter at least one value in each row for each data set). You cannot perform repeated measures ANOVA if there are any missing values. If you have averaged your data elsewhere, you may format the data table to enter mean, SD/SEM and N. The N values do not have to all be the same, but you cannot leave N blank. You cannot perform repeated measures twoway ANOVA if you enter averaged data. Some programs expect you to enter two-way ANOVA data in an indexed format. All the data are in one column, and two other columns are used to denote different levels of the two factors. You cannot enter data in this way into Prism. Prism can import indexed data with a single index variable, but cannot import data with two index variables, as would be required for twoway ANOVA. As with one-way ANOVA, it often makes sense to transform data before analysis, to make the distributions more Gaussian. See Advantages of transforming the data first on page 66. Choosing the two-way ANOVA analysis Start from the data or results table you wish to analyze (see "Entering data for two-way ANOVA" on page 93). Click Analyze and choose built-in analyses. Then choose two-way ANOVA from the list of statistical analyses Entering data for two-way ANOVA Arrange your data so the data sets (columns) represent different levels of one factor, and different rows represent different levels of the other factor. For example, to compare three time points in men and women, enter your data like this: Two-way analysis of variance 89 Analyzing Data with GraphPad Prism 90 Copyright (c) 1999 GraphPad Software Inc.

36 Variable names Label the two factors to make the output more clear. If you don t enter names, Prism will use the generic names Column factor and Row factor. Repeated measures You should choose a repeated measures analysis when the experiment used paired or matched subjects. See "Repeated measures test?" on page 67. Prism can calculate repeated measures two-way ANOVA with matching by either row or column, but not both. This is sometimes called a mixed model. The table above shows example data testing the effects of three doses of drugs in control and treated animals. The decision to use repeated measures depends on the experimental design. Here is an experimental design that would require analysis using repeated measures by row: The experiment was done with six animals, two for each dose. The control values were measured first in all six animals. Then you applied a treatment to all the animals, and made the measurement again. In the table above, the value at row 1, column A, Y1 (23) came from the same animal as the value at row 1, column B, Y1 (28). The matching is by row. Here is an experimental design that would require analysis using repeated measures by column: The experiment was done with four animals. First each animal was exposed to a treatment (or placebo). After measuring the baseline data (dose=zero), you inject the first dose and make the measurement again. Then inject the second dose and measure again. The values in the first Y1 column (23, 34, 43) were repeated measurements from the same animal. The other three columns came from three other animals. The matching was by column. The term repeated measures is appropriate for those examples, because you made repeated measurements from each animal. Some experiments involve matching but no repeated measurements. The term randomized block experiments describes this kind of experiments. For example, imagine that the three rows were three different cell lines. All the Y1 data came from one experiment, and all the Y2 data came from another experiment performed a month later. The value at row 1, column A, Y1 (23.0) and the value at row 1, column B, Y1 (28.0) came from the same experiment (same cell passage, same reagents). The matching is by row. Randomized block data are analyzed identically to repeated measures data. Prism only uses the term repeated measures. It is also possible to design experiments with repeated measures in both directions. Here is an example: The experiment was done with two animals. First you measured the baseline (control, zero dose). Then you injected dose 1 and made the next measurement, then dose 2 and measured again. Then you gave the animal the experimental treatment, waited an appropriate period of time, and made the three measurements again. Finally, you repeated the experiment with another animal (Y2). So a single animal provided data from both Y1 columns (23, 34, 43 and 28, 41, 56). Prism cannot perform two-way ANOVA with repeated measures in both directions, and so cannot analyze this experiment. Beware of matching within treatment groups -- that does not count as repeated measures. Here is an example: The experiment was done with six animals. Each animal was given one of two treatments at one of three doses. The measurement was then made in duplicate. The value at row 1, column A, Y1 (23) came from the same animal as the value at row 1, column A, Y2 (24). Since the matching is within a treatment group, it is a replicate, not a repeated measure. Analyze these data with ordinary twoway ANOVA, not repeated measures ANOVA. Two-way analysis of variance 91 Analyzing Data with GraphPad Prism 92 Copyright (c) 1999 GraphPad Software Inc.

### If you need statistics to analyze your experiment, then you've done the wrong experiment.

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### Comparing two groups (t tests...)

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