# Statistical Models in R

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1 Statistical Models in R Some Examples Steven Buechler Department of Mathematics 276B Hurley Hall; Fall, 2007

2 Outline Statistical Models Structure of models in R Model Assessment (Part IA) Anova in R

3 Statistical Models First Principles In a couple of lectures the basic notion of a statistical model is described. Examples of anova and linear regression are given, including variable selection to find a simple but explanatory model. Emphasis is placed on R s framework for statistical modeling.

4 General Problem addressed by modelling Given: a collection of variables, each variable being a vector of readings of a specific trait on the samples in an experiment. Problem: In what way does a variable Y depend on other variables X 1,..., X n in the study. Explanation: A statistical model defines a mathematical relationship between the X i s and Y. The model is a representation of the real Y that aims to replace it as far as possible. At least the model should capture the dependence of Y on the X i s

5 General Problem addressed by modelling Given: a collection of variables, each variable being a vector of readings of a specific trait on the samples in an experiment. Problem: In what way does a variable Y depend on other variables X 1,..., X n in the study. Explanation: A statistical model defines a mathematical relationship between the X i s and Y. The model is a representation of the real Y that aims to replace it as far as possible. At least the model should capture the dependence of Y on the X i s

6 General Problem addressed by modelling Given: a collection of variables, each variable being a vector of readings of a specific trait on the samples in an experiment. Problem: In what way does a variable Y depend on other variables X 1,..., X n in the study. Explanation: A statistical model defines a mathematical relationship between the X i s and Y. The model is a representation of the real Y that aims to replace it as far as possible. At least the model should capture the dependence of Y on the X i s

7 The Types of Variables in a statistical model The response variable is the one whose content we are trying to model with other variables, called the explanatory variables. In any given model there is one response variable (Y above) and there may be many explanatory variables (like X 1,....X n ).

8 Identify and Characterize Variables the first step in modelling Which variable is the response variable; Which variables are the explanatory variables; Are the explanatory variables continuous, categorical, or a mixture of both; What is the nature of the response variable is it a continuous measurement, a count, a proportion, a category, or a time-at-death?

9 Identify and Characterize Variables the first step in modelling Which variable is the response variable; Which variables are the explanatory variables; Are the explanatory variables continuous, categorical, or a mixture of both; What is the nature of the response variable is it a continuous measurement, a count, a proportion, a category, or a time-at-death?

10 Identify and Characterize Variables the first step in modelling Which variable is the response variable; Which variables are the explanatory variables; Are the explanatory variables continuous, categorical, or a mixture of both; What is the nature of the response variable is it a continuous measurement, a count, a proportion, a category, or a time-at-death?

11 Identify and Characterize Variables the first step in modelling Which variable is the response variable; Which variables are the explanatory variables; Are the explanatory variables continuous, categorical, or a mixture of both; What is the nature of the response variable is it a continuous measurement, a count, a proportion, a category, or a time-at-death?

12 Types of Variables Determine Type of Model The explanatory variables All explanatory variables continuous All explanatory variables categorical Explanatory variables both continuous and categorical Regression Analysis of variance (Anova) Analysis of covariance (Ancova)

13 Types of Variables Determine Type of Model The response variable what kind of data is it? Continuous Proportion Count Binary Time-at-death Normal Regression, Anova, Ancova Logistic regression Log linear models Binary logistic analysis Survival analysis

14 Model Formulas Which variables are involved? A fundamental aspect of models is the use of model formulas to specify the variables involved in the model and the possible interactions between explanatory variables included in the model. A model formula is input into a function that performs a linear regression or anova, for example. While a model formula bears some resemblance to a mathematical formula, the symbols in the equation mean different things than in algebra.

15 Common Features of model formulas Model formulas have a format like > Y ~ X1 + X2 + Z * W where Y is the explanatory variable, means is modeled as a function of and the right hand side is an expression in the explanatory variables.

16 First Examples of Model Formulas Given continuous variables x and y, the relationship of a linear regression of y on x is described as > y ~ x The actual linear regression is executed by > fit <- lm(y ~ x)

17 First Examples of Model Formulas If y is continuous and z is categorical we use the same model formula > y ~ z to express that we ll model y as a function of z, however in this case the model will be an anova, executed as > fit <- aov(y ~ z)

18 Multiple Explanatory Variables Frequently, there are multiple explanatory variables invovled in a model. The + symbol denotes inclusion of additional explanatory variables. The formula > y ~ x1 + x2 + x3 denotes that y is modeled as a function of x1, x2, x3. If all of these are continuous, > fit <- lm(y ~ x1 + x2 + x3) executes a mutliple linear regression of y on x1, x2, x3.

19 Other Operators in Model Formulas In complicated relationships we may need to include interaction terms as variables in the model. This is common when a model involves multiple categorical explanatory variables. A factorial anova may involve calculating means for the levels of variable A restricted to a level of B. The formula > y ~ A * B describes this form of model.

20 Just the Basics Here, just the basic structure of modeling in R is given, using anova and linear regression as examples. See the Crawley book listed in the syllabus for a careful introduction to models of varying forms. Besides giving examples of models of these simple forms, tools for assessing the quality of the models, and comparing models with different variables will be illustrated.

21 Outline Statistical Models Structure of models in R Model Assessment (Part IA) Anova in R

22 Approximate Y The goal of a model is to approximate a vector Y with values calculated from the explanatory variables. Suppose the Y values are (y 1,..., y n ). The values calculated in the model are called the fitted values and denoted (ŷ 1,..., ŷ n ). (In general, a hat on a quantity means one approximated in a model or through sampling.) The goodness of fit is measured with the residuals, (r 1,..., r n ), where r i = y i ŷ i.

23 Measure of Residuals Two obtain a number that measures the overall size of the residuals we use the residual sum of squares, defined as RSS = n (y i ŷ i ) 2. i=1 As RSS decreases ŷ becomes a better approximation to y.

24 Residuals: Only Half of the Story A good model should have predictive value in other data sets and contain only as many explanatory variables as needed for a reasonable fit. To minimize RSS we can set ŷ i = y i, for 1 i n. However, this model may not generalize at all to another data set. It is heavily biased to this sample. We could set ŷ i = ȳ = (y y n )/n, the sample mean, for all i. This has low bias in that other samples will yield about the same mean. However, it may have high variance, that is a large RSS.

25 Residuals: Only Half of the Story A good model should have predictive value in other data sets and contain only as many explanatory variables as needed for a reasonable fit. To minimize RSS we can set ŷ i = y i, for 1 i n. However, this model may not generalize at all to another data set. It is heavily biased to this sample. We could set ŷ i = ȳ = (y y n )/n, the sample mean, for all i. This has low bias in that other samples will yield about the same mean. However, it may have high variance, that is a large RSS.

26 Residuals: Only Half of the Story A good model should have predictive value in other data sets and contain only as many explanatory variables as needed for a reasonable fit. To minimize RSS we can set ŷ i = y i, for 1 i n. However, this model may not generalize at all to another data set. It is heavily biased to this sample. We could set ŷ i = ȳ = (y y n )/n, the sample mean, for all i. This has low bias in that other samples will yield about the same mean. However, it may have high variance, that is a large RSS.

27 Bias-Variance Trade-off Selecting an optimal model, both in the form of the model and the parameters, is a complicated compromise between minimizing bias and variance. This is a deep and evolving subject, although it is certainly settled in linear regression and other simple models. For these lectures make as a goal minimzing RSS while keeping the model as simple as possible. Just as in hypothesis testing, there is a statistic calculable from the data and model that we use to measure part of this trade-off.

28 Statistics Measure the Fit Comparing two models fit to the same data can be set up as a hypothesis testing problem. Let M 0 and M 1 denote the models. Consider as the null hypothesis M 1 is not a significant improvement on M 0, and the alternative the negation. This hypothesis can often be formulated so that a statistic can be generated from the two models.

29 Model Comparison Statistics Normally, the models are nested in that the variables in M 0 are a subset of those in M 1. The statistic often involves the RSS values for both models, adjusted by the number of parameters used. In linear regression this becomes an anova test (comparing variances). More robust is a likelihood ratio test for nested models. When models are sufficiently specific to define a probability distribution for y, the model will report the log-likelihood, ˆL. Under some mild assumptions, 2(ˆL 0 ˆL 1 ) follows a chi-squared distribution with degrees of freedom = difference in number of parameters on the two models.

30 Comparison with Null Model The utility of a single model M 1 is often assessed by comparing it with the null model, that reflects no dependence of y on the explanatory variables. The model formula for the null model is > y ~ 1 signifying that we use a constant to approximate y. The natural constant is the mean of y. Functions in R that generate models report the statistics that measure it s comparison with the null model.

31 Outline Statistical Models Structure of models in R Model Assessment (Part IA) Anova in R

32 Continuous Factors Analysis of variance is the modeling technique used when the response variable is continuous and all of the explanatory variables are categorical; i.e., factors. Setup: A continuous variable Y is modeled against a categorical variable A. Anova Model Structure: On each level approximate the Y values by the mean for that level.

33 Continuous Factors Analysis of variance is the modeling technique used when the response variable is continuous and all of the explanatory variables are categorical; i.e., factors. Setup: A continuous variable Y is modeled against a categorical variable A. Anova Model Structure: On each level approximate the Y values by the mean for that level.

34 Anova Model Structure Suppose Y is the vector (y 1,..., y m ). In an anova model the fit value for y i in level A j is the mean of the y values in level A j. So, the fit vector is a vector of level means. The null model for an anova uses mean(y) to approximate every y i. An anova model with two levels is basically a t test. The t test assesses whether it is statistically meaningful to consider the group means as different, or approximate both the global mean.

35 Setup and Assumptions for a simple anova Consider the continuous variable Y, partioned as y ij, the j th observation in factor level i. Suppose there are K levels. Assumptions: The anova is balanced, meaning every level has the same number n of elements. Let Y i = { y ij : 1 j n } Each Y i is normally distributed. The variances of the Y i s are constant.

36 Setup and Assumptions for a simple anova Consider the continuous variable Y, partioned as y ij, the j th observation in factor level i. Suppose there are K levels. Assumptions: The anova is balanced, meaning every level has the same number n of elements. Let Y i = { y ij : 1 j n } Each Y i is normally distributed. The variances of the Y i s are constant.

37 Setup and Assumptions for a simple anova Consider the continuous variable Y, partioned as y ij, the j th observation in factor level i. Suppose there are K levels. Assumptions: The anova is balanced, meaning every level has the same number n of elements. Let Y i = { y ij : 1 j n } Each Y i is normally distributed. The variances of the Y i s are constant.

38 Sums of Squares measure the impact of level means Several sums of squares help measure the overall deviation in the Y values, the deviation in the model and the RSS. Let ȳ i be the mean of Y i, ȳ the mean of Y (all values). SSY = SSA = RSS = SSE = K n (y ij ȳ) 2 i=1 j=1 K (ȳ i ȳ) 2 i=1 K i=1 j=1 n (y ij ȳ i ) 2

39 The Statistic to Assess Fit is F The statistic used to assess the model is calculated from SSA and SSE by adjusting for degrees of freedom. Let MSA = SSA/(K 1) and MSE = SSE/K(n 1). Define: F = MSA MSE. Under all of the assumptions on the data, under the null hypothesis that all of the level means are the same, F satisfies an F distribution with K 1 and K(n 1) degrees of freedom.

40 Anova in R An anova in R is executed with the aov function. The sample data have a vector Y, of values and an associated factor LVS of three levels, each containing 20 samples. As usual, it should be verified that the values on each level are normally distributed and there is a constant variance across the levels. First visualize the relationship between Y and LVS with a boxplot. > plot(lvs, Y, main = "Boxplot Anova Sample Data") > abline(h = mean(y), col = "red")

41 Plot of Sample Data Boxplot Anova Sample Data A B C

42 Execute Anova and Summarize > aovfit1 <- aov(y ~ LVS) > summary(aovfit1) Df Sum Sq Mean Sq F value Pr(>F) LVS e-06 *** Residuals Signif. codes: 0 '***' '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 Conclude: there is a significant difference in level means.

43 Plots Assessing the Anova Model > plot(aovfit1) Fitted values Residuals aov(y ~ LVS) Residuals vs Fitted

44 Variables May be Components Frequently, the variables on which a statistical model is generated are components in a data frame. If the data frame dat has components HT and FLD, an anova could be executed as > aovfld <- aov(ht ~ FLD, data = dat) All statistical modeling formulas have a data optional parameter.

45 Underlying Linear Model Actually, this analysis of variance is a form of multiple linear regression, with a variable for each level in the factor. Many features of modeling are the same and R reflects this.

46 (Intercept) LVSB *** > summary.lm(aovfit1) Call: aov(formula = Y ~ LVS) Underlying Linear Model Residuals: Min 1Q Median 3Q Max Coefficients: Estimate Std. Error t value Pr(> t ) (Intercept) LVSB e-05 LVSC e-06

47 Getting (Almost) Real The assumptions of a classical anova aren t realistic. Various refined methods handle unbalanced data (levels of different sizes), non-normally distributed data, multiple nested factors, readings within a factor collected over time (longitudinal data), and pseudo-replication. The types of models to reference are: split-plot, random effects, nested design, mixed models. These methods can require significant care in defining the model formula and interpreting the result.

48 Getting (Almost) Real The assumptions of a classical anova aren t realistic. Various refined methods handle unbalanced data (levels of different sizes), non-normally distributed data, multiple nested factors, readings within a factor collected over time (longitudinal data), and pseudo-replication. The types of models to reference are: split-plot, random effects, nested design, mixed models. These methods can require significant care in defining the model formula and interpreting the result.

49 Getting (Almost) Real The assumptions of a classical anova aren t realistic. Various refined methods handle unbalanced data (levels of different sizes), non-normally distributed data, multiple nested factors, readings within a factor collected over time (longitudinal data), and pseudo-replication. The types of models to reference are: split-plot, random effects, nested design, mixed models. These methods can require significant care in defining the model formula and interpreting the result.

50 Kruskal-Wallis Test Just as an anova is a multi-level t test, the Kruskal-Wallis test is a multi-level version of the Mann-Whitney test. This is a non-parametric test that does not assume normality of the errors. It sums rank as in Mann-Whitney. For example, the Kruskal-Wallis test applied to the earlier data is executed by > ktest <- kruskal.test(y ~ LVS) ktest will be an htest object, like that generated by t.test.

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