Introduction to the Analysis of Variance (ANOVA)

Transcription

1 Introduction to the Analysis of Variance (ANOVA)

2 The Analysis of Variance (ANOVA) The analysis of variance (ANOVA) is a statistical technique for testing for differences between the means of multiple (more than two) groups It is probably the most prevalent statistical technique used in psychological research. The ANOVA is a flexible technique that can be used with a variety of different research designs. In today s lecture, I will explain the logic behind the ANOVA and introduce the one-way between groups ANOVA, which is an ANOVA in which the groups are defined along only one independent (or quasi-independent) variable

3 The Analysis of Variance The purpose of ANOVA is much the same as the t tests presented in the preceding lectures Are the mean differences obtained for sample data sufficiently large for us to conclude that there are mean differences between the populations from which the samples were obtained The difference between ANOVA and the t tests is that ANOVA can be used in situations where there are two or more means being compared, whereas the t tests are limited to situations where only two means are involved.

4 Instructor 1 Instructor 2 Instructor 3 Populations (µ,σ unknown) Samples

5 The Problem of Multiple Comparisons The ANOVA is necessary to protect researchers from an excessive experimentwise error rate in situations where a study is comparing more than two population means. Experimentwise error rate: the probability of making at least one Type I error across mutliple comparisons These situations would require a series of several t tests to evaluate all of the mean differences. (Remember, a t test can compare only two means at a time) So? Why not just use multiple t-tests?

6 The Problem of Multiple Comparisons Why not just use multiple t-tests? Although each t test can be evaluated using a specific α-level (risk of Type I error), the α-levels accumulate over a series of tests so that the final familywise α-level can be quite large Example: For 5 levels of the independent variable, there are 10 possible pairwise comparisons between group means: {1,2},{1,3},{1,4},{1,5},{2,3},{2,4},{2,5},{3,4},{3,5},{4,5}

7 The Problem of Multiple Comparisons Assume H 0 is true and α=0.05. Then the probability of accepting H 0 in a single pairwise comparison is: P accept single pairwise H However, we have to make 10 such comparisons. Using the multiplicative law of probability (remember that?), and assuming independent pairwise tests, the probability of correctly retaining the null in all 10 comparisons is: accept all P H Therefore, experiment 1 P accept all H We now have a 40% overall chance of making a Type I error! 0

8 Intro to ANOVA

9 Null and Alternative Hypotheses in ANOVAs The omnibus null hypothesis is the null hypothesis in the ANOVA: that the population means of all groups being compared are equal i.e., for three groups, H 0 : μ 1 = μ 2 = μ 3 Alternative Hypothesis: at least one population mean is different from the others.

10 Omnibus Null Hypothesis: µ 1 = µ 2 = µ 3

11 The Logic of the Analysis of Variance The test statistic for ANOVA is an F-ratio, which is a ratio of two sample variances. F variance including any treatment effects variance without any treatment effects MS MS between within In the context of ANOVA, the sample variances are called mean squares, or MS values The numerator, MS between, measures the size of mean differences between samples from different treatment groups The denominator, MS within (or MS error ), measures the magnitude of differences that would be expected without any treatment effects

12 The Logic of the Analysis of Variance Total Variance Between Treatments Variance Measures differences caused by: Systematic treatment effects Sampling error Within Treatments Variance Measures differences caused by: Sampling error

13 Assumptions of the ANOVA Normality of Scores I.e., we assume that the scores in all of our group populations are normally distributed Since this is important primarily for the sampling distribution of the mean, the ANOVA is fairly robust to violations of this assumption, especially if the sample sizes are reasonably large Homogeneity of variances We assume that each population of scores has the same variance E.g., [error variance] ANOVA is fairly robust to violations of this assumption Independence of observations E.g., given the population parameters, knowing one person s score tells you nothing about another person s score. Violations of this assumption can have serious implications for an analysis.

14 The Logic of the ANOVA Regardless of whether or not the null hypothesis is true, the assumption of homogeneity of variances implies that all population variances are equal Thus, as we did for the independent-samples t-test, we can estimate this shared population variance by taking the average of the sample variances (the pooled variance),, s s s n s Avg s1 s s ˆwithi p (assuming n 1 = n 2 = n 3 )

15 The Logic of the ANOVA However, if all the population means are equal (under H 0 ), then we have a second way to estimate the population variance we can estimate the population variance using the variance of the sample means Recall that the Central Limit Theorem tells us how to compute the variance of sample means from the population variance: 2 2 M n We can rearrange this formula to solve for the population variance given the variance of sample means: n 2 2 M

16 The Logic of the ANOVA Of course, we don t have the variance of sample means either. However, we can estimate it by computing the variance of our three group means s Var M, M, M 2 2 ˆM M Plugging this into the previous equation, our second estimate of the population variance is ns 2 2 ˆbetween M

17 The Logic of the ANOVA We now have two estimates of the population variance: An estimate computed from the sample variances, which should estimate the population variance regardless of whether H 0 is true ˆwith,, in s p Avg s1 s2 s 3 A second estimate computed from the sample means, which only estimates the population variance if H 0 is true 2 2 ˆ between nsm nvar M1, M 2, M 3

18 The Logic of the ANOVA The F-ratio used as the test statistic for the ANOVA is simply the ratio between these two estimates of the population variance F 2 MS ˆ 1,, between nvar M M M between MS ˆ within within Avg s1, s2, s 3 If H 0 is true, then these two estimates should be equal (on average) In this case, the ratio should be 1.0 However, if H 0 is false, then the estimate in the numerator (which is based on the variability of sample means) will include the treatment effect in addition to differences in sample means expected by chance In this case, the ratio should be greater than

19 The F distribution reject H 0 retain H 0

20 Populations Samples

21 The Logic of the ANOVA Sample 1 Sample 2 Sample 3 n = 20 n = 20 n = 20 M = 65.4 M = M = 71.2 s 2 = s 2 = s 2 = M sum M 2 ˆ s 2 2 within p s1 s2 s ns 2 2 ˆbetween M SS n df M M M T SS M nm M n k 3 M M k = ˆbetween 215 F 2 ˆ within