Art Kendall wrote:
>In SPSS output ignore the lines for equal variances, and use the lines for
>unequal variances.
Last year on this group, there was an interesting dataset posted, in which the
equal and unequal variance t tests give very different results:
Temperatures from Portion 1 of a stream:
16.9
17
15.8
17.1
18.7
18
mean = 17.25
variance = 0.995
Portion 2
18.3
18.5
mean = 18.4
variance = 0.02
The SPSS unequal variance t-test gives a 2-tailed P of 0.037, but the equal
variance t produces a two-tailed P of 0.174
An exact test is possible with these data, as there are only 28 ways of
forming groups of 6 and 2, and only 3 of these groupings produce a difference
in means equal to or greater than the observed (one being the observed, p=0.11,
reasonably close to the equal variance t test). I gave this example to my
class, just so they would not automatically use the unequal variance t test
output (it is certainly not appropriate for unequal group sizes if the smaller
group has the smaller variance)
Gene Gallagher
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