Re: Repeated measures analysis of fractions summing to a constant

Posted by Kornbrot, Diana on Apr 05, 2013; 7:53am
URL: http://spssx-discussion.165.s1.nabble.com/Repeated-measures-analysis-of-fractions-summing-to-a-constant-tp5719257p5719276.html

SPSS generalized model does indeed provide appropriate routines for analyzing count data

Use either GEE generalized Estimating Equation or Genealized linear models
Your model is for COUNTS – recommend negative binomial with log link. The % should be converted to number of hours as response variable, participants is a subject variable, and category [work, meal, etc is rpeated measures]. Model has category as predictor and shold also be entered under fixed.

Have fun

Diana


On 05/04/2013 04:43, "Kirill Orlov" <kior@...> wrote:

 Ryan,
 For example, the DV might be "how do you spend your typical day?" question
 Work __% of time
 Meals__% of time
 Stroll__% of time
 Comp/TV/Reading__% of time
 Else__% of time
 [Please check that your answers sum to 100%]
 
 Converting to logits might be interesting idea, although not necessarily most right. But I wonder if SPSS (GEE or other procedure) have already forseen and provided tools (reference distribution + link function) exactly for a DV which is fractions summing to a constant; for such a DV isn't uncommon.
 
 
 
05.04.2013 2:32, Subscribe SAS-L Anonymous пишет:
 
 

Would the OP mind explaining exactly what the DV is?   It might help shed light on how to proceed, notwithstanding other interesting solutions. Speaking of which, the idea of converting the probs to logits is intriguing. I am generally in favor of the logit scale because of its properties, but the fact that it addresses the linear dependence is an added bonus here.

Anyway, I would appreciate if the OP would be willing to tell us more about the DV.

Ryan

On Apr 4, 2013, at 4:05 AM, Kirill Orlov <kior@...> <[hidden email]>  wrote:

 

Consider you have a between-within design: several between-subject groups and several (3 or more) repeated measures (= within-subject) trials. It's all very classic and typical. The nuance, however, is that the values for every subject sum across the repeated levels to a **constant**. This is because the data are complementary, i.e. percentages of fractions, so, in this case they sum to 100 for every individual. For example, with 3 RM levels, a respondent's data is like 30%, 22%, 48% (sum=100); for another respondent 25%, 33%, 42% (sum=100).

I know that I can analyze between-groups X repeated-measures count data via Generalized Estimating Equations procedure. By I doubt in this case because the values *sum to a constant*, they are complementary fractions; they are not counts of successes in repeated independent trials!

Can I analyze such data in SPSS and how? Thanks.

 

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