Copyright (c) Bani K. Mallick1 STAT 651 Lecture #13.

Copyright (c) Bani K. Mallick 1

STAT 651

Lecture #13

Topics in Lecture #13 Multiple comparisons, especially

Fisher’s Least Significant Difference

Residuals as a means of checking the normality assumption

Book Sections Covered in Lecture #13

Chapter 8.4 (Residuals)

Chapter 9.4 (Fisher’s)

Chapter 9.1 (the idea of multiple comparisons)

Lecture 12 Review: ANOVA

Suppose we form three populations on the basis of body mass index (BMI):

BMI < 22, 22 <= BMI < 28, BMI > 28

This forms 3 populations

We want to know whether the three populations have the same mean caloric intake, or if their food composition differs.

One procedure that is often followed is to do a preliminary test to see whether there are any differences among the populations

Then, once you conclude that some differences exist, you allow somewhat more informality in deciding where those differences manifest themselves

The first step is the ANOVA F-test

The distance of the data to the overall mean is

TSS = (Corrected) Total Sum of Squares

This has degrees of freedom

TSS = (Y Y )

The sum of squares between groups Corrected Model) is

It has t-1 degrees of freedom, so the number of populations is the degrees of freedom between groups + 1.

n (Y Y )

The distance of the observations to their sample means is

This is the Sum of Squares for Error

It has degrees of freedom

SSE = (Y Y )

Next comes the F-statistic

It is the ratio of the mean square for the corrected model to the mean square for error

Large values indicate rejection of the null hypothesis Tests of Between-Subjects Effects

Dependent Variable: Baseline FFQ

960.287a 2 480.143 5.689 .004

196009.919 1 196009.919 2322.508 .000

960.287 2 480.143 5.689 .004

15275.639 181 84.396

226223.216 184

16235.925 183

SourceCorrected Model

Intercept

BMIGROUP

Corrected Total

Type III Sumof Squares df Mean Square F Sig.

R Squared = .059 (Adjusted R Squared = .049)a.

The F-statistic is compared to the F-distribution with t-1 and degrees of freedom.

See Table 8 ,which lists the cutoff points in terms of . If the F-statistic exceeds the cutoff, you reject the hypothesis of equality of all the means.

SPSS gives you the p-value (significance level) for this test

The F-statistic is compared to the F-distribution with df1 = t-1 and degrees of freedom.

For example if you have 3 populations, 6 observations for each population, then there are 18 total observations.

The degrees of freedom are 2 and 15. If you want a type I error of 5%, look at df1 = 2, df2 = 15, = .05 to get a critical value of 3.68: try this out!

2 Tdf =n t

If the populations have a common variance 2, the Mean squared error estimates it.

You take the square root of the MSE to estimate

Tests of Between-Subjects Effects

960.287a 2 480.143 5.689 .004

196009.919 1 196009.919 2322.508 .000

960.287 2 480.143 5.689 .004

15275.639 181 84.396

226223.216 184

16235.925 183

Intercept

BMIGROUP

Corrected Total

The critical value of 2 and 181 df for an F-test at Type I error 0.05 is about 3.05

Hence F > 3.05, so the p-value is < 0.05

960.287a 2 480.143 5.689 .004

196009.919 1 196009.919 2322.508 .000

960.287 2 480.143 5.689 .004

15275.639 181 84.396

226223.216 184

16235.925 183

Intercept

BMIGROUP

Corrected Total

ANOVA in SPSS

“Analyze”, “General Linear Model”, “Univariate”

“Fixed factor” = the variable defining the populations

Always “Save” unstandardized residuals

“Posthoc”: Move factor to right and click on LSD

ANOVA Table

960.287a 2 480.143 5.689 .004

196009.919 1 196009.919 2322.508 .000

960.287 2 480.143 5.689 .004

15275.639 181 84.396

226223.216 184

16235.925 183

Intercept

BMIGROUP

Corrected Total

Fisher’s Least Significant Distance (LSD)

Suppose that we determine that there are at least some differences among t population means.

Fisher’s Least Significant Difference is one way to tell which ones are different

The main reason to use it is convenience: all comparisons can be done with the click of a mouse

It does not guarantee longer or shorter confidence intervals

For example, suppose there are t = 3 populations.

The null hypothesis is

The alternative is:

But this does not tell you which populations are different, only that some are

0 1 2 3Η :μ =μ =μ

0H : null hypothesis is false

The null hypothesis is

The alternative is:

There are 4 possibilities:

Fishers LSD is a way of getting this directly

0 1 2 3H :μ =μ =μ

0H : null hypothesis is false

Fisher’s LSD

We have done an ANOVA, and now we want to compare two specific populations.

Fisher’s LSD differs from our usual 2-population comparisons in two features:

The degrees of freedom (nT-t) not n1+n2-2

The pooled standard deviation (square

root of MSE = SSE/(nT-t) , not sP

Review: Comparing Two Populations

If you can reasonably believe that the population sd’s are nearly equal, it is customary to pick the equal variance assumption and estimate the common standard deviation by

2 21 1 2 2

(n 1)s (n 1)ss

n +n 2

Comparing Two Populations: Usual and Fisher LSD

1 2X X /2 1 1 2

2 pt (n +n -2n

1 2Tn t

1 1MSE

Fisher

ROS Data

ROS data has three groups: Fish oil diet, Fish-like oil diet, and Corn Oil

We want to compare their responses to butyrate

Between-Subjects Factors

FAEE oildiet

Fish oil diet 10

Corn oil diet 10

DietGroup

Value Label N

ROS data, log scale. What do you see?

101010N =

ROS Response After Butyrate Exposure

Diet Group

Corn oil dietFish oil dietFAEE oil diet

ROS data, log scale. What do you see? Maybe different variances, but sample sizes are small

101010N =

Diet Group

ROS data, log scale. No major changes in means?

101010N =

Diet Group

ROS data has three groups: Fish oil diet, Fish-like oil diet, and Corn Oil

What was the total sample size? n = 30Tests of Between-Subjects Effects

Dependent Variable: log(Butyrate) - log(Control)

5.188E-02a 2 2.594E-02 .203 .818

5.957 1 5.957 46.542 .000

5.188E-02 2 2.594E-02 .203 .818

3.456 27 .128

9.465 30

3.508 29

Intercept

DIETGRP

Corrected Total

R Squared = .015 (Adjusted R Squared = -.058)a.

ROS data: any evidence that the population means are different in their change after butyrate exposure?

5.188E-02a 2 2.594E-02 .203 .818

5.957 1 5.957 46.542 .000

5.188E-02 2 2.594E-02 .203 .818

3.456 27 .128

9.465 30

3.508 29

Intercept

DIETGRP

Corrected Total

ROS data: any evidence that the population means are different in their change after butyrate exposure? No, the p-value is 0.818!

This matches the box plotsTests of Between-Subjects Effects

5.188E-02a 2 2.594E-02 .203 .818

5.957 1 5.957 46.542 .000

5.188E-02 2 2.594E-02 .203 .818

3.456 27 .128

9.465 30

3.508 29

Intercept

DIETGRP

Corrected Total

ROS Data

Testing for Normality in ANOVA

I use the General Linear Model to define these residuals

Form the residuals, which are simply the differences of the data with their group sample mean

Then do a q-q plot

Useful if you have many groups with a small number of observations per group

Here is the Q-Q plot. How’s it look?

ROS: log scale

Observed Value

1.0.50.0-.5-1.0

ROS Data

Testing for Normality in ANOVA:

Illustrate saving residuals: “general linear model”, “univariate”, “save” (select “unstandardized” to create the residual variable )

Illustrate q-q- plot on residuals

Illustrate editing a chart object to change titles and the like

ROS Data Fisher’s LSD. Note how all p-values are

> 0.10.

Multiple Comparisons

6.825E-02 .1600 .673 -.2600 .3965

9.960E-02 .1600 .539 -.2287 .4279

-6.8255E-02 .1600 .673 -.3965 .2600

3.135E-02 .1600 .846 -.2969 .3596

-9.9605E-02 .1600 .539 -.4279 .2287

-3.1350E-02 .1600 .846 -.3596 .2969

(J) Diet GroupFish oil diet

Corn oil diet

FAEE oil diet

Corn oil diet

FAEE oil diet

Fish oil diet

(I) Diet GroupFAEE oil diet

Fish oil diet

Corn oil diet

MeanDifference

(I-J) Std. Error

Pvalues

Sig. Lower Bound Upper Bound

95% Confidence Interval

Based on observed means.

ROS Data: Compare Fish to Corn oil

Mean for fish – mean for corn =

6.825E-02 .1600 .673 -.2600 .3965

9.960E-02 .1600 .539 -.2287 .4279

-6.8255E-02 .1600 .673 -.3965 .2600

3.135E-02 .1600 .846 -.2969 .3596

-9.9605E-02 .1600 .539 -.4279 .2287

-3.1350E-02 .1600 .846 -.3596 .2969

Corn oil diet

FAEE oil diet

Corn oil diet

FAEE oil diet

Fish oil diet

Corn oil diet

MeanDifference

(I-J) Std. Error

Pvalues

Mean for fish – mean for corn = 0.03135

Standard error =

6.825E-02 .1600 .673 -.2600 .3965

9.960E-02 .1600 .539 -.2287 .4279

-6.8255E-02 .1600 .673 -.3965 .2600

3.135E-02 .1600 .846 -.2969 .3596

-9.9605E-02 .1600 .539 -.4279 .2287

-3.1350E-02 .1600 .846 -.3596 .2969

Corn oil diet

FAEE oil diet

Corn oil diet

FAEE oil diet

Fish oil diet

Corn oil diet

MeanDifference

(I-J) Std. Error

Pvalues

Standard error = 0.1600

CI (95%) = Multiple Comparisons

6.825E-02 .1600 .673 -.2600 .3965

9.960E-02 .1600 .539 -.2287 .4279

-6.8255E-02 .1600 .673 -.3965 .2600

3.135E-02 .1600 .846 -.2969 .3596

-9.9605E-02 .1600 .539 -.4279 .2287

-3.1350E-02 .1600 .846 -.3596 .2969

Corn oil diet

FAEE oil diet

Corn oil diet

FAEE oil diet

Fish oil diet

Corn oil diet

MeanDifference

(I-J) Std. Error

Pvalues

Standard error = 0.1600

CI (95%) = -2969 to .3596Multiple Comparisons

6.825E-02 .1600 .673 -.2600 .3965

9.960E-02 .1600 .539 -.2287 .4279

-6.8255E-02 .1600 .673 -.3965 .2600

3.135E-02 .1600 .846 -.2969 .3596

-9.9605E-02 .1600 .539 -.4279 .2287

-3.1350E-02 .1600 .846 -.3596 .2969

Corn oil diet

FAEE oil diet

Corn oil diet

FAEE oil diet

Fish oil diet

Corn oil diet

MeanDifference

(I-J) Std. Error

Pvalues

Concho Water Snake Illustration

A numerical example will help illustrate this idea. I’ll consider comparing tail lengths of female Concho Water Snakes with age classes 2,3, and 4.

Sample sizes

Sample sd:

Sample means:

1 2 3 Tn 11,n 17,n 9,n 37.

1 2 3s 17.90,s 10.95,s 13.58.

1 2 3153.82, 173.24, 194.67.

Female Concho Water Snakes, Ages 2-4, Tail Length

Between-Subjects Factors

91711N =

4.003.002.00

Female Concho Water Snakes, Ages 2-4, Tail Length: are they different in population means?

Dependent Variable: Tail Length

8269.413a 2 4134.706 21.304 .000

1043505.649 1 1043505.649 5376.698 .000

8269.413 2 4134.706 21.304 .000

6598.695 34 194.079

1118093.000 37

14868.108 36

Intercept

Corrected Total

Concho Water Snake Example

Dependent Variable: Tail Length

-19.4171 * 5.3907 .001 -30.3724 -8.4618

-40.8485 * 6.2616 .000 -53.5736 -28.1233

19.4171 * 5.3907 .001 8.4618 30.3724

-21.4314 * 5.7429 .001 -33.1023 -9.7604

40.8485 * 6.2616 .000 28.1233 53.5736

21.4314 * 5.7429 .001 9.7604 33.1023

(J) Age3.00

(I) Age2.00

MeanDifference

(I-J) Std. Error Sig. Lower Bound Upper Bound

The mean difference is significant at the .05 level.*.

Concho Water Snake Illustration: Hand Calculations

Sample size factor for comparing the age groups

Sample mean difference

1 10.41

43.2123

Concho Water Snake Illustration

nT – t = 34 degrees of freedom for error

MSE = 194.08,

= 0.05

= 9.76 to 33.10: compare with output

MSE 13.93 2 Tn t 2.03t

3 2 2 T2 3

1 1n t MSE

43.2123

Normal Q-Q Plot of Residual for TAILL

Observed Value

3020100-10-20-30-40

We need a method that allows for non-normal data!

Copyright (c) Bani K. Mallick1 STAT 651 Lecture #13.

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