Dear all,
As part of the output for a t-test, SPSS performs a test of equal
variances and then gives two results for the t-test depending on
whether this first test fails or not.
My question is whether you should base the equality assumption
on this test or if you should decide beforehand whether the
two populations would be expected to have equal variances.
Consider the situation where one of the groups is very small
- the estimate of the variance in this small group may be rather
poor with the result the equal-variance test fails (especially
if the combined sample size of 10,000 is swamping the test with power).
Any thoughts?
cheers
Jon
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Jon Heron, PhD
Research Statistician
Avon Longitudinal Study of Parents and Children
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