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Generalizing to a Population:
REPEATED-MEASURES MODELS continued

 Repeated-Measures ANOVA with three or more trials plus a between-subjects effect

model: numeric <= (subject)  trial  group  trial*group
example: jumphgt <= (athlete)  time  group  time*group
You should be able to see that this model is the previous two merged together. The interpretations of the main effects and interaction term from the first of the previous models (two time points, two groups) still apply. And all the problems with sphericity from the model on the previous page (three or more trials in one group of subjects) still have to be addressed.

To summarize:

• Use univariate ANOVA with adjustment for non-sphericity, or model the covariance matrix with proc mixed and/or a statistician.
• Trial*group tells you how the experiment worked.
• Contrasts between different levels of trial and group for the trial*group term tell you where the experimental group differs from the control or other groups.
• Get the program to give you estimates and confidence intervals, not just contrasts and p values.

If you didn't know any better, you might try to analyze these data by doing a series of unpaired t tests for each time point. That would be foolish, for three reasons: the power to detect differences would be lousy, because you would not be making use of changes in each subject's values; you would not be taking into account any differences between the two groups at baseline; and finally you would not impress the reviewers and editor of the journal you submit the research to. You could fix the first two criticisms by subtracting each subject's mean baseline value from the post1 and post2 values, then doing unpaired t tests on these difference scores.

Special Case: Multiple Crossovers and Latin Squares
Recall that a simple crossover is a design in which all subjects receive two treatments. We analyzed the data with a treatment effect and a group effect that indicated which treatment each subject got first:

model: numeric <= (subject)  treat  group  treat*group
example: jumphgt <= (athlete)  treat  group  treat*group

OK, but what about more than two treatments? For example, can we have a crossover in which every subject gets a control treatment and two experimental treatments aimed at increasing jump height? Sure, just use the above model. Treat is the repeated-measures or within-subject effect, with three or more levels. The group effect represents the sequence of treatments that each subjects is assigned to, and it also has three or more levels, as we'll see. And in the same manner as for a simple crossover, with SAS you can use a trial effect instead of a group effect to indicate whether each treatment was first, second, third... for each subject (see earlier):

model: numeric <= (subject)  treat  trial  treat*trial

Whichever model you use, the good news is that you still need only about a quarter of the subjects of a fully controlled study! Here's how to set up and analyze these multiple crossovers.

First, randomize your subjects to the various sequences of treatments. For example, if you have two experimental treatments (A and B), and a control treatment (C), there are six possible sequences: A-B-C, A-C-B, B-C-A, B-A-C, C-A-B, and C-B-A. It's best to use all these sequences, because if one of the treatments has a carry-over effect, it will affect all the other treatments equally.

Next, decide on sample size. By running a simulation for this design, I've found that about 12 subjects give acceptable confidence limits for the pairwise comparisons of the treatment effects, for very high reliability (r=0.95). So you could start with two subjects doing each of the six sequences of treatments, then do more subjects if necessary, as described in sample size on the fly. If one or two subjects miss a treatment, or if you lose one or two subjects completely, no great problem: the data don't have to be "balanced" to give unbiased estimates, provided none of the treatments carry over.

Now do the work, get the data, and analyze them. If you can use proc mixed in SAS, I recommend the crossover model with trial: it gives slightly better confidence intervals, and it's easy to estimate the learning effects from the levels of the trial effect (e.g. for three treatments there's one learning effect between first and second trials and another between second and third trials). See the simulation for the program statements. Non-SAS users will have to use the usual crossover model with a group effect, but it's fiendishly difficult to work out the appropriate combination of levels of treat*group to get the learning effects.

How best to plot data for a multiple crossover? For a simple crossover I suggested showing the means for the two groups of subjects for control and experimental treatments. That approach is no good for multiple treatments, because there'll be too many groups and not enough subjects in each group. One solution is simply to plot the means for each treatment. Connect all the points together, as I have done in the figure, to show they all have the same repeated-measures relationship to each other. Or if the treatments can be put into a sensible order, such as an increasing dose of something, plot the treatments in order along the X axis and connect the points sequentially. Either way, you shouldn't use a bar graph.

Standard deviations are a problem with multiple crossovers. You could plot the standard deviations for each treatment, but they will be inflated by any learning effects. Stats wizards using proc mixed in SAS can extract the composite between-subject SD from the ANOVA. This SD includes within-subject retest error, but it is not affected by the treatment and learning effects. It's therefore the best measure of variation by which to assess visually the magnitude of the treatment effects shown in your plot.

In the text of the Results section, give the raw differences between the means for the treatments and the confidence intervals for these differences. Or when appropriate (e.g. for most athletic performance measures), show percent differences and their confidence intervals, as provided by analysis of log-transformed performance measures.

When there are four treatments altogether (e.g. a control and three experimental treatments), there are 24 possible sequences of treatments. You could randomize one subject to each sequence of treatments, but you might not need 24 subjects to get acceptable confidence limits for your comparisons. But with random assignment of less than 24 subjects, you might not end up with balance in the way treatments follow each other. A problem if one of the treatments has a carry-over effect, because it will have the greatest effect on the treatment that follows it most times. So it's better to randomize to a subset of sequences that ensures every treatment follows every other treatment the same number of times. Any carry-over effect will then affect every other treatment equally. Such a balanced subset of sequences is called a Latin square. Here's the Latin-square set for four treatments, A, B, C, and D:

Sequence 1: A B C D
Sequence 2: B D A C
Sequence 3: D C B A
Sequence 4: C A D B

Check and you'll see that each treatment follows every other treatment only once. Your sample size obviously has to be a multiple of 4 to keep the balance. For example, for 12 subjects, assign three at random to each of the four sequences.

Here's a balanced set of sequences for five treatments. In this case you need 10 sequences to keep the balance (and each treatment is followed by every other treatment twice), so you'll need multiples of 10 subjects in your study:

1: A B C D E      6: E D C B A
2: B D A E C      7: C E A D B
3: D E B C A      8: A C B E D
4: E C D A B      9: B A D C E
5: C A E B D     10: D B E A C

We're into seldom-trodden territory now, but I must record the trick of generating these Latin squares, in case you want to do more than five treatments. Instead of labeling the treatments with letters (A, B, C...), let's label them with numbers (1, 2, 3...). Assume n treatments. Here is the Latin square for n even. As above, the sequences of treatments are given by the rows, not the columns:

 1 2 n 3 n-1 4 n-2 . . 2 3 1 4 n 5 n-1 . . 3 4 2 5 1 6 n . . . . . . . . . . . . . . . . . . . .

For n odd, use the above set of sequences, plus its mirror image. Check out the sets for five treatments given above, to see what I mean. Thanks, Dennis Loiselle!

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Last updated 11 May 98