
Re: Testing the significance of difference in set means at different times
Posted:
Nov 1, 2012 10:12 AM


On 01/11/2012 6:27 AM, Stuart.Palmer@deakin.edu.au wrote: > On Thursday, 1 November 2012 17:47:30 UTC+11, Ray Koopman wrote: >> On Oct 31, 9:16 pm, Stuart.Pal...@deakin.edu.au wrote: >> >>> I have data from two large sets of samples (100s in one set [a1] and 1000s in the other set [b1]). a1 has a mean score Ma1. b1 has a mean score Mb1. I can evaluate the significance associated with the difference in mean scores [Ma1Mb1] observed between these two sets. >> >>> >> >>> I also have two similar sets of data collected at a later period  a2 and b2, with means Ma2 and Mb2. Again, I can evaluate the significance associated with the difference in mean scores [Ma2Mb2] observed between these two sets. >> >>> >> >>> I'm interested in whether there is a way to evaluate the significance of the observed change across the two time periods between the difference observed in the set means? That is, an appropriate method to evaluate the significance of the observed difference in (Ma1Mb1)(Ma2Mb2). >> >>> >> >>> Any sugestions would be greatly appreciated. >> >> >> >> For each person, get the algebraic difference between their score >> >> at time 1 and their score at time 2. Then do an independentgroups >> >> ttest of the difference between the means of the two sets of >> >> difference scores. > > > Thanks for the reply. > > I omitted an important piece of information sorry, each of the sets is a crosssectional sample (subset) of their respective populations, and the data in a1 and a2 (and b1 and b2) are not paired.
When you say the observations are not paired, that could mean that each person (or unit of observation, if not people) appeared in one and only one of the 4 samples. Or it could mean that people could appear in more than one sample, but you don't have the information needed to pair up their observations. Which is it? If the former, you have 4 independent groups in a 2x2 layout, and you can use twoway ANOVA (Condition x Time). The null hypothesis for the interaction term is that the difference between A and B is the same at both time points. (It could also be stated that the difference between times 1 and 2 is the same for conditions A and B.)
You can find some nice introductory notes on twoway (or factorial) ANOVA here: http://davidmlane.com/hyperstat/  see chapter 13.
HTH.
 Bruce Weaver bweaver@lakeheadu.ca http://sites.google.com/a/lakeheadu.ca/bweaver/Home "When all else fails, RTFM."

