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Re: Discrepancy between ttest and regression analysis outcome?
Posted:
May 21, 2006 11:56 AM


On 5/21/2006 5:48 AM, TLW wrote: > I have been working on the data analysis of a group of data for two > different (though related) problems. > First we were interested in determining whether the data could be > considered homogeneous. > Secondly we were interested in predictions based on the data. > > So far I have assumed that those two problems should give 'similar' > answers. I.e.: if a ttest shows that groups A and B do not differ; > then a regression test using the data in group A should give (more or > less) the same model as a regression test with group B. > > However, in my data this is not true.
<...snip...>
> * Am I right that the identification of 'no difference' between two > positions should mean that those two positions should give > (approximately) the same model using stepwise regression?
Without going into detail about what you did (cause I didn't understand everything), when you do a ttest of differences, and the result is no difference, that means that the MEAN of the two groups is not significantly different. It says nothing about the regression itself. The regressions can be dramatically different around the same mean. You should be able to draw an example on a piece of paper...
You might want to fit a regression to each group and test to see if the groups have the same intercept and test to see if the groups have the same slope.
 Paige Miller pmiller5@rochester.rr.com
It's nothing until I call it  Bill Klem, NL Umpire If you get the choice to sit it out or dance, I hope you dance  Lee Ann Womack



