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AJones
‎09-01-2015
  • 4 Posts
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Re: PROC TTEST versus PROC GLM - CONTRAST

by Calcite | Level 5 in SAS Procedures
‎05-23-2013 12:05 PM
‎05-23-2013 12:05 PM
Thank you.  I thought I was restricting to the two levels in GLM by using a WHERE statement, but in fact I had not thought to delete the 7 extra zero placeholders in the CONTRAST statement.  That must have been throwing it off, because the probability does indeed now match the TTEST.  What you say about the additional CLASS variable certainly makes sense. As far as defining/combining variable levels, I agree that this should not be done post-experiment.  In this case, I'm doing a meta-analysis and looking for trends to determine whether certain conditions are significant across studies--I am neutral to whether they are combined or not, but will certainly comment on it either way in my analysis. I appreciate your thoughtful response and explanation! ... View more

Re: PROC TTEST versus PROC GLM - CONTRAST

by Calcite | Level 5 in Statistical Procedures
‎05-23-2013 11:47 AM
‎05-23-2013 11:47 AM
Thank you.  I thought I was restricting to the two levels by using a WHERE statement, but in fact I had not thought to delete the 7 extra zero placeholders in the CONTRAST statement.  That must have been throwing it off, because the probability does indeed now match the TTEST.  I appreciate your response! ... View more

PROC TTEST versus PROC GLM - CONTRAST

by Calcite | Level 5 in Statistical Procedures
‎05-22-2013 04:05 PM
‎05-22-2013 04:05 PM
I'm running tests on a dataset where my CLASS variable has 9 levels.  Two of the levels are very similar, and I want to determine whether they are significantly different from each other to see if they can actually be separated or whether they need to be combined. When I run PROC TTEST restricted to these two levels, they are shown to be significantly different from each other (Pr > |t|  is  <0.0001) and (Pr > F is <0.0001). When I run a PROC GLM modeling the same 'var' variable for these two levels, however, the CONTRAST statement returns a significant yet different result (Pr > F = 0.0407).  Should I expect TTEST and CONTRAST to find the same significance?  Is it recommended to rely on one or the other when testing for a significant difference? Also, if I run the full model (with an additional CLASS variable), the Pr > F in the CONTRAST output for the two levels increases to 0.3231.  Would you think that the levels need to be significantly different from each other in this full model, or only in the more basic model?  I realize that perhaps this decision may be left to the discretion of the modeler. ... View more

PROC TTEST versus PROC GLM - CONTRAST

by Calcite | Level 5 in SAS Procedures
‎05-22-2013 02:20 PM
‎05-22-2013 02:20 PM
I'm running tests on a dataset where my CLASS variable has 9 levels.  Two of the levels are very similar, and I want to determine whether they are significantly different from each other to see if they can actually be separated or whether they need to be combined. When I run PROC TTEST restricted to these two levels, they are shown to be significantly different from each other (Pr > |t|  is  <0.0001) and (Pr > F is <0.0001). When I run a PROC GLM modeling the same 'var' variable for these two levels, however, the CONTRAST statement returns a significant yet different result (Pr > F = 0.0407).  Should I expect TTEST and CONTRAST to find the same significance?  Is it recommended to rely on one or the other when testing for a significant difference? Also, if I run the full model (with an additional CLASS variable), the Pr > F in the CONTRAST output for the two levels increases to 0.3231.  Would you think that the levels need to be significantly different from each other in this full model, or only in the more basic model?  I realize that perhaps this decision may be left to the discretion of the modeler. ... View more
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