A Guide for a Selection of SPSS Functions

Transcription

1 A Guide for a Selection of SPSS Functions IBM SPSS Statistics 19 Compiled by Beth Gaedy, Math Specialist, Viterbo University Using documents prepared by Drs. Sheldon Lee, Marcus Saegrove, Jennifer Sadowski and Michael Alfieri

2 TABLE OF CONTENTS How to enter data into SPSS data editor: General instructions... 3 How to create figures in SPSS: General instructions... 4 Creating Bar Graphs... 5 Creating Pie Charts... 7 Creating Histograms... 7 Computing the Mean, Median, and Standard Deviation Creating Box Plots To make a Normal Quantile Plot Make a Scatter Plot Standard Pearson correlation a.k.a Pearson product-moment correlation: Linear Regression Find the regression equation Make a Scatter Plot with regression line in SPSS Residual Plot in SPSS Spearman rank-order correlation Logistic regression Paired T-test Independent T-test Mann Whitney U a.k.a Wilcoxon-Mann-Whitney ANOVA / General Linear Model Post-Hoc tests ANOVA Kruskal-Wallis test Chi-square test of association (Contingency Tables) Goodness of Fit Test

3 How to enter data into SPSS data editor: General instructions The Data Editor provides two views of your data: Data View. This view displays the actual data values or defined value labels (resembles an excel spreadsheet) Variable View. This view displays variable definition information, including defined variable and value labels, data type (for example, string, date, or numeric), measurement level (nominal, ordinal, or scale), and userdefined missing values. 3

4 In both views, you can add, change, and delete information that is contained in the data file. You can switch between them at the tabs located at the bottom left hand of the page. Variable View contains descriptions of the attributes of each variable in the data file. In Variable View: rows are variables and columns are variable attributes. Type in the name of your variable (no spaces), choose the type of data (by clicking the grey box in the cell), a label for your variable (spaces are allowed this will be the label that shows up on figures/graphs). If desired, in the values box code for your values by clicking on the grey box in the cell, and then coding 0=male, 1=female. When you then return to data view, your codes will translate to your values (do this under view and values labels ). Once your variables are set up in variable view, return to data view and enter your data. Remember to save both your data file and your data output separately (they are different file types). How to create figures in SPSS: General instructions General instructions: To create a graph/figure on SPSS, go to Graphs, then Legacy Dialogs and choose the appropriate option. Typical options are: Bar and Scatter/dot. There are many options for graphs and/or details (e.g., error bars, best fit lines, etc) consult the Help function on SPSS for additional detailed instructions. 4

5 Creating Bar Graphs For categorical data, we may use bar graphs or pie charts to visualize the data. In the data below, the three columns are the favorite colors, favorite football teams, and pulse rates for students in a class. These are categorical data, so we may use a bar graph or pie chart to display the data. To make a bar graph in SPSS, go to Graphs Legacy Dialogs Bar. Note that the Simple bar graph is the default. Click Define. Choose either simple or clustered ( Clustered is for grouping data for example, by weeks). Drag one of the categorical variables (such as Favorite Color) onto the Category Axis. Click OK to finish. Notice: You can add additional Titles and change what the Bars Represent. 5

6 This is the resulting graph, which can be cut and pasted into other documents. Also from the Bars Represent area: - Choose your variable of interest and click other statistic from the Bars Represent area. The mean is automatically chosen (displayed on y- axis). Include the variable of interest for your x- axis and include it in the category box. 6

7 Creating Pie Charts Graphs Legacy Dialogs Pie and click on Define. Drag a categorical variable into the Define Slices by box. (NOTE: Here the results will be shown as a % rather than an N value.) Creating Histograms For one set of data: METHOD #1 (the simple way) Graphs Legacy Dialogs Histogram 7

8 From the column listings, drag the appropriate variable to Variable. METHOD #2 (more editing options) Graphs Chart Builder 8

9 This shot shows that a simple histogram was chosen, and the data for Volume Diet Cola was placed into the x-axis. The Element Properties window automatically appears. TO SET CUSTOM VALUES FOR ANCHORS AND BINS: In the Set Parameters tab of the Element Properties window, you can use the Custom value for anchor" option to determine the starting endpoint for the bins. To change the bin width, find "Bin Sizes", select Custom, click on Interval Width, and enter the desired bin width into the box 9

10 Histograms, continued: For two sets of data: You need one column for the actual quantitative data, and another column used to specify which group the data belongs to. Select the right-most icon (back-to-back histogram) and drag it to the chart preview area. Drag the quantitative variable into Distribution Variable box, and drag the grouping variable into the Split Variable box. Computing the Mean, Median, and Standard Deviation Enter all the values into a column. Select Analyze Descriptive Statistics Frequencies. Select the variables desired and drag them to the Variable(s) panel. 10

11 Click the Statistics tab and choose what you want to calculate. (If you want to add Percentile(s), type in the value and click Add.) Click continue to return to the Frequencies window, and click OK to compute. Here is the output: Age Statistics N Valid 54 Missing 0 Mean Median Std. Deviation Percentiles Creating Box Plots There are several ways to make box plots in SPSS, some of which are described below. Single box plot Graphs Chart Builder, select Boxplot from the Gallery, drag the rightmost icon into the chart preview area. Drag the corresponding variable into the x-axis, then click OK to finish. Side-by-side box plots, data in separate columns Analyze Descriptive Statistics Explore, drag all variables to plot into the Dependent List. Click Plots. Under Boxplots, select Dependents together. You also have the option to plot stem-and-leaf plots and histograms. Side-by-side box plots, data in a single column with another column that is the grouping variable. Graphs Chart Builder, select Boxplot from the Gallery, drag the leftmost icon into the chart preview area. Drag the variable onto the y-axis, and the grouping variable onto the x-axis. To make a Normal Quantile Plot Analyze Descriptive Statistics Explore Drag the appropriate column or columns into "Dependent List" Click "Plots" Check "Normality Plot with tests" Optional: check Histogram to plot a histogram. Under boxplots, select Factor levels together to make a boxplot for each variable, or select Dependents together to make side-by-side boxplots. Click "Continue" Click "OK" Look for the Normal Q-Q Plots in the Output window (You may ignore the Detrended Normal Q-Q plots) 11

12 Make a Scatter Plot Graphs LegacyDialogs Scatter/Dot Select Simple Scatter, click Define Drag the appropriate columns to the X-axis and Y-axis. Standard Pearson correlation a.k.a Pearson product-moment correlation: Tests whether there is an association between two variables Data (for both variables) are continuous, parametric Example: There is a significant association between plant height and plant root length (Pearson correlation: r=0.75, n = 20, p=0.02) Pearson correlation How to enter data: Input data into columns add appropriate labels Pearson correlation - How to run test: Analyze Correlate Bivariate move variables into variables box check Pearson and Two-tailed Ok Linear Regression Determines relationship between two variables and implies that a prediction of one value is being attempted from another (i.e., cause and effect). Data are Continuous NOTE: x = cause, predictor, or independent variable that is set or chosen by the experimenter y = effect, dependent which is never set by the experimenter Example: The uptake of drug X is significantly affected by ph level: uptake increases at higher ph levels (linear regression: p<0.001) Linear Regression How to enter data: Input data into two columns label columns appropriately Linear Regression How to run test: Analyze Regression Linear move the effect variable into the dependent box move the cause variable into the independent box Ok 12

13 Find the regression equation (continued from steps above) Optional: Click Save. To save the predicted values, check Unstandardized under predicted Values. To save the residuals, check Unstandardized under Residuals. Click OK The bottom table in the Output window contains the coefficients. In this case, the regression equation is If the value is sufficiently close to 1, you may use the regression equation to do predictions. For example, for somebody who has had 6.5 beers, the approximated blood alcohol level would be Make a Scatter Plot with regression line in SPSS Graphs LegacyDialogs Scatter/Dot Select Simple Scatter, click Define Drag the appropriate columns to the X-axis and Y-axis and click OK In the Output window, double-click on the scatterplot. Select Elements Fit Line at Total. Set the Fit Method to Linear The best-fit line is drawn, and the value is shown to the right of the plot. To convert from the value to the value, you take the positive or negative square root, depending on whether or not the correlation is positive or negative. Residual Plot in SPSS Analyze Regression Linear Enter dependent variable (y column) and independent variable (x column) appropriately Click Save. To save the residuals, check Unstandardized under Residuals. Click OK, a new column is displayed (probably called RES_1) Make a regular scatterplot putting the independent variable on the x-axis and the residual on the y- axis. 13

14 Spearman rank-order correlation Tests whether there is an association between two variables Data are discrete, non-parametric Example: There is not a significant association between male and female body size in pairs of penguins (Spearman correlation: r s =0.771, n=20, p=0.072) NOTE Spearman rank-order is the non-parametric equivalent of the Pearson correlation (pg 83) Spearman rank-order correlation How to enter data: Enter all data into two columns o Enter category labels o Enter value labels Spearman rank-order correlation How to enter data: Select Analyze correlate then Bivariate Highlight both variables and move them into Variables box Make sure that Spearman is selected, and that the test of Significance is Two-tailed Click OK Logistic regression Regression, as above, implies cause and effect when y, dependent variable ( effect ) is classified into groups NOTE: Dependent (effect, y, never set by experimenter): only classified into groups Independent (cause, x, is set or chosen by experimenter): can be continuous OR can be in groups Example: Shade level has a significant effect on the presence of the plant virus (logistic regression: p=0.006) Logistic regression How to enter data: 2 columns 1 st has treatment as yes or no (0 or 1) 2 nd has category groupings (1-?) Logistic regression How to run test: Analyze Regression Binary Logistic move 'effect' variable to "Dependent" box Place 'cause' variable in "Covariates" box "OK" 14

15 Tests of Differences (for each know when to use, what type of data, how to write results): Paired T-test Data are continuous, paired, in 2 groups, parametric Example: Plants in treatment A were significantly taller ( = 5.6 cm) after 7 days than plants in treatment B ( = 4.5 cm) (t-test: p= value of p, Figure 1). Paired T-test How to enter data: Arrange data in 2 columns Columns labeled as before and after Should be one individual per row Paired T-test How to run test: Analyze Compare means Paired samples t-test choose variables click ok Independent T-test Data are continuous, unpaired, in 2 groups, parametric Example: Plants in treatment A were significantly taller ( = 5.6 cm) after 7 days than plants in treatment B ( = 4.5 cm) (t-test: p= value of p, Figure 1). Independent T-test How to enter data : 1 st column put in all data 2 nd column assign category to each data Go to Variable View- under "Values" put in code 1 or 2 Independent T-test How to run test : Analyze Compare Means Independent Samples T Test Place independent variable in appropriate box Grouping variable in other, press "define groups," put in 1 and 2 "Continue" "OK" Read line with "Equal variances assumed" line for appropriate p-value 15

16 Mann Whitney U a.k.a Wilcoxon-Mann-Whitney Data are discrete, unpaired, in 2 groups, non-parametric Example: There were significantly more plants in treatment A than in treatment B (Mann- Whitney U test: p= 0.025) NOTE Mann-Whitney U is the non-parametric equivalent to the independent samples t-test Mann Whitney U How to enter data: Enter collected data into a column Enter categories (as numbers) in next column o Go to variable view and enter in corresponding information, including the values column as what which # represents in the data view. Mann Whitney U How to run test: Select Analyze Nonparametric Tests 2-Independent Samples (by default, the Mann-U should be selected) Enter dependent variable in the Test Variable List Enter independent variable in the Grouping Variable Define Groups (define them as 1 and 2) Click OK to run ANOVA / General Linear Model Data are continuous, unpaired, in 2 groups or more, parametric Example: There is a significant difference in grain size among the three cultivars (ANOVA: F=11.879, df=2, p=0.001) ANOVA How to enter data: All the data are entered into one column with a second column for the labeling of the groups. ANOVA How to run test: Analyze menu choose Compare Means One-way ANOVA. the data (numerical) variable goes into the Dependent box the grouping variable goes into the Factor box. By clicking on the Options menu you can request a means plot, some descriptive statistics of the data, and a test for Homogeneity of variance. Click continue, then OK. GLM How to enter data: Create columns [categorical/variable] enter data into appropriate columns label variables within columns if necessary 16

17 GLM How to run test: Analyze General Linear Model Univariate move dependent variable into appropriate box move fixed factor into the appropriate box Ok ANOVA / General Linear Model Post-Hoc tests ANOVA Data are continuous, unpaired, in more than 2 groups, parametric Example: There is a significant difference in grain size among the three cultivars (ANOVA: F=11.879, df=2, p=0.001). Premier is significantly smaller than super or dupa and there is no difference between super and dupa (least significant difference (LSD) post-hoc test, table 1). ANOVA How to enter data: See directions for ANOVA ANOVA How to run test: Run as ANOVA While selecting variables, click post hoc button Select LSD, SNK Click ok in display box Click ok GLM How to enter data in SPSS: similar to ANOVA GLM How to run test in SPSS: similar to ANOVA Kruskal-Wallis test Data are discrete, unpaired, in more than 2 groups, non-parametric Example: There is a significant difference in grain size of the three cultivars (Kruskal Wallis: p=0.013). NOTE Kruskal-Wallis is the non-parametric equivalent to the one-way ANOVA NOTE If Kruskal-Wallis results state a significant difference, then pairwise Mann-Whitney U tests can be run. Kruskal-Wallis How to enter data: Use independent t-test to enter data but this time have a 3 rd group instead of just 2 Kruskal-Wallis How to run test: Analyze Non-parametric Tests K Independent Samples make sure "Kruskal-Wallis test is selected (default) 17

18 Place data into the "test variable list" Grouping variable in other, press "define groups" put in min. and max (ex: if there are 3 groups, min=1 and max=3) "Continue" "OK" Chi-square test of association (Contingency Tables) Tests whether there is an association between two variables Data are categorical, non-parametric Example: There was a significant association between stream velocity category and stream bed category (Chi-square test: X 2 = , d.f. = 3, p=0.012) Chi-square test of association How to enter data: Create 3 columns in the worksheet: frequency, row, and column. The frequency column contains the data, and the row and column columns identify the location of the data in the matrix. Chi-square test of association How to run test: Data menu Choose Weight Cases Click frequency into frequency variable box. Click OK. Analyze menu choose Descriptive Statistics select Crosstabs Enter row into row and column into column. Click Statistics select Chi-square Continue. click Cells in the options box select both Observed and Expected in the Counts area. click Continue OK in the Crosstabs box. Click Display clustered bar charts to produce a visual summary of the frequencies in each of the categories. Goodness of Fit Test Enter categories (by number) in one column, and frequency in adjacent column. DataWeight Cases; click Weight Cases by and insert frequency column in Frequency Variable: AnalyzeNonparametric TestsLegacy DialogChi Square Test Click category column into Test Variable List Click appropriate case in Expected Values ; add proportions in Values (if appropriate) Click OK Chi-Square value, d.f. and P-value appear in output. 18