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Analysis of variance 

This article discusses the analysis of variance (ANOVA) method for comparing more than two groups in situations where there is one observation per object. It explains the concept of variance within and between groups, and provides an example and SPSS output for conducting a one-way ANOVA. The Kruskal-Wallis test, a non-parametric alternative to ANOVA, is also mentioned. Additionally, it suggests when to use each method based on the data characteristics.

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Analysis of variance 

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  1. Analysis of variance  Tron Anders Moger 2006.31.10

  2. Comparing more than two groups • Up to now we have studied situations with • One observation per object • One group • Two groups • Two or more observations per object • We will now study situations with one observation per object, and three or more groups of objects • The most important question is as usual: Do the numbers in the groups come from the same population, or from different populations?

  3. ANOVA • If you have three groups, could plausibly do pairwise comparisons. But if you have 10 groups? Too many pairwise comparisons: You would get too many false positives! • You would really like to compare a null hypothesis of all equal, against some difference • ANOVA: ANalysis Of VAriance

  4. One-way ANOVA: Example • Assume ”treatment results” from 13 patients visiting one of three doctors are given: • Doctor A: 24,26,31,27 • Doctor B: 29,31,30,36,33 • Doctor C: 29,27,34,26 • H0: The means are equal for all groups (The treatment results are from the same population of results) • H1: The means are different for at least two groups (They are from different populations)

  5. Comparing the groups • Averages within groups: • Doctor A: 27 • Doctor B: 31.8 • Doctor C: 29 • Total average: • Variance around the mean matters for comparison. • We must compare the variance within the groups to the variance between the group means.

  6. Variance within and between groups • Sum of squares within groups: • Compare it with sum of squares between groups: • Comparing these, we also need to take into account the number of observations and sizes of groups

  7. Adjusting for group sizes • Divide by the number of degrees of freedom • Test statistic: reject H0 if this is large Both are estimates of population variance of error under H0 n: number of observations K: number of groups

  8. Test statistic thresholds • If populations are normal, with the same variance, then we can show that under the null hypothesis, MSG and MSW are Chi-square distributed with K-1 and n-K d.f. • Reject at confidence level if The F distribution, with K-1 and n-K degrees of freedom Find this value in table p. 871

  9. Continuing example • Thus we can NOT reject the null hypothesis in our case.

  10. ANOVA table NOTE:

  11. Formulation of the model: • H0: µ1=µ2=…=µK • Xij=µi+εij • Let Gi be the difference between the group means and the population mean. Then: • Gi=µi-µ of µi=µ+Gi • Giving Xij=µ+Gi+εij • And H0: G1=G2=…=GK=0

  12. One-way ANOVA in SPSS Last column: The p-value: The smallest value of at which the null hypothesis is rejected.

  13. One-way ANOVA in SPSS: • Analyze - Compare Means - One-way ANOVA • Move dependent variable to Dependent list and group to Factor • Choose Bonferroni in the Post Hoc window to get comparisons of all groups • Choose Descriptive and Homogeneity of variance test in the Options window

  14. Energy expenditure example: • Let us say we have measurements of energy expenditure in three independent groups: Anorectic, lean and obese • Want to test H0: Energy expenditure is the same for anorectic, lean and obese • Data for anorctic: 5.40, 6.23, 5.34, 5.76, 5.99, 6.55, 6.33, 6.21

  15. SPSS output: • See that there is a difference between groups. • See also between which groups the difference is!

  16. Conclusion: • There is a significant overall difference in energy expenditure between the three groups (p-value<0.001) • There are also significant differences for all two-by-two comparisons of groups

  17. The Kruskal-Wallis test • ANOVA is based on the assumption of normality • There is a non-parametric alternative not relying this assumption: • Looking at all observations together, rank them • Let R1, R2, …,RK be the sums of ranks of each group • If some R’s are much larger than others, it indicates the numbers in different groups come from different populations

  18. The Kruskal-Wallis test • The test statistic is • Under the null hypothesis, this has an approximate distribution. • The approximation is OK when each group contains at least 5 observations.

  19. Example: previous data (We really have too few observations for this test!)

  20. Kruskal-Wallis in SPSS • Use ”Analyze=>Nonparametric tests=>K independent samples” • For our data, we get

  21. For the energy data: • Same result as for one-way ANOVA! • Reject H0

  22. When to use what method • In situations where we have one observation per object, and want to compare two or more groups: • Use non-parametric tests if you have enough data • For two groups: Mann-Whitney U-test (Wilcoxon rank sum) • For three or more groups use Kruskal-Wallis • If data analysis indicate assumption of normally distributed independent errors is OK • For two groups use t-test (equal or unequal variances assumed) • For three or more groups use ANOVA

  23. When to use what method • When you in addition to the main observation have some observations that can be used to pair or block objects, and want to compare groups, and assumption of normally distributed independent errors is OK: • For two groups, use paired-data t-test • For three or more groups, we can use two-way ANOVA

  24. Two-way ANOVA (without interaction) • In two-way ANOVA, data fall into categories in two different ways: Each observation can be placed in a table. • Example: Both doctor and type of treatment should influence outcome. • Sometimes we are interested in studying both categories, sometimes the second category is used only to reduce unexplained variance (like an independent variable in regression!). Then it is called a blocking variable • Compare means, just as before, but for different groups and blocks

  25. Data from exercise 17.46: • Three types of aptitude tests (K=3) given to prospective management trainers • Each test type is given to members of each of four groups of subjects (H=4): Profile fit, Mindbender, Psych Out

  26. Sums of squares for two-way ANOVA • Assume K groups, H blocks, and assume one observation xij for each group i and each block j, so we have n=KH observations (independent!). • Mean for category i: • Mean for block j: • Overall mean: • Model: Xij=µ+Gi+Bj+εij

  27. Sums of squares for two-way ANOVA

  28. ANOVA table for two-way data Test for between groups effect: compare to Test for between blocks effect: compare to

  29. Two-way ANOVA (with interaction) • The setup above assumes that the blocking variable influences outcomes in the same way in all categories (and vice versa) • We can check if there is interaction between the blocking variable and the categories by extending the model with an interaction term • Need more observations per block • Other advantages: More precise estimates

  30. Data from exercise 17.46 cont’d: • Each type of test was given three times for each type of subject

  31. Sums of squares for two-way ANOVA (with interaction) • Assume K groups, H blocks, and assume L observations xij1, xij2, …,xijL for each category i and each block j block, so we have n=KHL observations (independent!). • Mean for category i: • Mean for block j: • Mean for cell ij: • Overall mean: • Model: Xijl=µ+Gi+Bj+Iij+εijl

  32. Sums of squares for two-way ANOVA (with interaction)

  33. ANOVA table for two-way data (with interaction) Test for interaction: compare MSI/MSE with Test for block effect: compare MSB/MSE with Test for group effect: compare MSG/MSE with

  34. Two-way ANOVA in SPSS • Analyze->General Linear Model-> Univariate • Move dependent variable (Score) to Dependent Variable • Move test type and subject type to Fixed Factor(s) • Under Options, may check Descriptive Statistics and Homogeneity Tests, and also get two-by-two comparisons by checking Bonferroni under Post Hoc • Gives you a full model (with interaction)

  35. Some SPSS output: Equal variances can be assumed • See that there is a significant block effect, significant group effect, and a significant interaction effect • Means (in plain words) that test score is different for subject types, for the three tests, and that difference for test type depends on what block you consider

  36. Two-by-two comparisons

  37. Notes on ANOVA • All analysis of variance (ANOVA) methods are based on the assumptions of normally distributed and independent errors • The same problems can be described using the regression framework. We get exactly the same tests and results! • There are many extensions beyond those mentioned • In fact, the book only briefly touches this subject • More material is needed in order to do two-way ANOVA on your own

  38. Next time: • How to design a study? • Different sampling methods • Research designs • Sample size considerations

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