when ns are = and var are the same ... or, when ns are = and var are NOT
the same ... or, when ns are NOT = but var are the same ... the stan error
for difference formulas for pooled and non pooled are the same ... when ns
are NOT = and var are NOT = ...the two standard errors are different ...
and i am not sure if the difference in the stan errors is always in one
direction ...
so, could be that for first case, stan error is smaller and second case, it
is larger ...
why are there only 2 observations in the second case ... but 6 in the first???
At 02:16 PM 11/3/00 -0500, Richard Lehman wrote:
>A colleague sent me this note.
>
>
>>A statistics question.
>>
>>Temperatures taken from different portions of a stream:
>>
>>Portion 1
>>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
>>
>>Do these portions have different temperatures?
>>
>>Obviously the variances are unequal and a 2-sample [unequal variance]
>
> t = 2.74 w/ 5 df p = 0.037.
>
>>No problem.
>
>
>>But (and this is where I am perplexed), a pooled [equal variance]
>
> t = 1.54 w/ 6 df p = 0.17.
>
>>Why is the less conservative pooled t giving a lower t-value? Are the
>>variances so uequal (and the one so close to zero) that the formula is
>>messed up?
>
>
>
>
>I have a hunch that his hunch is probably correct, if inexactly formulated.
>
>Any insights you can offer? Thanks.
>
>
>--------------------------------------------------------------------------
>
>Richard S. Lehman [EMAIL PROTECTED]
>Professor of Psychology (That's R-underscore-Lehman)
>Department of Psychology
>Franklin & Marshall College Voice (717)291-4202
>PO Box 3003 FAX (717)291-4387
>Lancaster, PA 17604-3003
> "I'd rather be blowing glass."
>
>
>
>
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