Do I really need to run ANOVA? However, an ANOVA test doesn’t tell you where you have the differences. Before talking about the differences of the comparisons, you need to know one important thing. Some of forex 101 knyga du post-hoc comparisons may not be appropriate for repeated-measure ANOVA.
For the discussion, let’s say you have three groups: A, B, and C. Compare every possible combination of your groups: In this case, you compare everything. You will compare A and B, B and C, and C and A. Compare every group: In this case, you will compare A and B, B and C, and C and A. Compare every group against the control: In this case, you will compare the groups only against the control condition. If your control condition is A, you will compare A and B, and A and C. Compare the data against the within-subject factor: In this case, you will make a comparison against the within-subject factor.
In other words, if you have done repeated-measure ANOVA, this is the way to do a post-hoc test. The methods for this case can be used for tests against between-subject factors. The strict definition of a post-hoc test means a test which does not require any plan for testing. In the four examples above, only Case 1 satisfies the strict definition of a post-hoc test because the others cases require some sort of planning on which groups to compare or not to compare. However, the term of a post-hoc test is often used for meaning Case 2 and Case 3. The key point here is you need to figure out which case you want to do for the comparisons after ANOVA.
Make sure what comparisons you are interested in before doing any test. This means that the effect of one factor depends on the conditions controlled by the other factors. So, what you need to do is to make comparisons against one factor with fixing the other factors. The effects we look at here are called simple main effects. Although you could report all the simple main effects you have, it might become lengthy. Rather, providing a graph like above is easier to understand what kinds of trends your data have.
Your graph will look like one of the following graphs depending of the existence of main effects and interactions. This is because significant differences are caused by some combination of the factors, and not necessarily caused by a single factor. However, I think that it is still good to report any significant main effects even if you have a significant interaction because it tells us what the data look like. The sad news is that it is not as easy in R to do post-hoc test as in SPSS. You often have to do a lot of things manually or some methods are not really supported in R. So, particularly if you are doing two-way ANOVA and you have an access to SPSS, it may be better to use SPSS. Scheffe’s test allows you to do a comparison on every possible combination of the group you are caring about.
Unfortunately, there is no automatic way to do Scheffe’s test in R. You have to do it manually. We use the same example for one-way ANOVA. The each element of coeff intuitively means the weight for the later calculation. The positive values and negative values represent data to compare. In this case, the first element is 1, and the second element is -1. And the first, second, and third elements will represent Group A, B, and C, respectively.
So, this coeff means we are going to compare Group A and Group B. Another key point is the sum of the elements must be 0. Let’s come back to the example. Now we need to calculate the number of the samples and the mean for each group. We then calculate some statistical values.