Multcompare for friedman structure
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Hi there,
Does anyone know what kind of pairwise test the multcompare function perform, when the input stats structure comes from a friedman test? On the description, only the t-test is mentioned, however this is not relevant to friedman tests which are usually employed under non-normality assumptions.
Multiple-comparison results also look a bit funny for friedman tests. For example, although a significant factor is discovered during the friedman test (p<1e-24) no significant differences come across during the multiple comparisons, -when it is obvious from the data that there are some-.
I've had a look at the multcompare.m source code, but couldn't figure what pairwise test is performed, although it seems to me that this is the same regardless of the stats.source.
Any help would be appreciated.
Agamemnon
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Jeremy Kemmerer
2014-10-1
Hi Agamemnon,
The function ‘multcompare’ should accept a ‘stats’ structure from the function ‘friedman’ without any problems. While it seems strange that you would see such a low p-value from the ‘friedman’ test but see no significant differences pair-wise, I believe this does happen in practice. Does the graph produced by ‘multcompare’ show groups that are far apart?
Also, by default, MATLAB uses a ‘tukey-kramer’ critical value for data sets generated from ‘friedman.’ This critical value can be changed by changing the ‘ctype’ parameter, although other values may be more conservative.
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