Date: Mon, 19 Dec 2011 12:50:10 -0500 Rich Ulrich "SPSSX(r) Discussion" Rich Ulrich Re: Can I compare two within-subjects' dependent variables while holding co-variates? To: noahermesh@gmail.com <1324311713513-5086603.post@n5.nabble.com> multipart/alternative;

If, as you say, you are new at this, then you certainly need to keep the analyses as simple as possible, because you have plenty of trouble in laying out simple *hypotheses* from this information and you don't need to add the difficulty of reading the tests on obscure contrasts that are only implicit -- Be explicit about what you are contrasting. See below for further comments.

> Date: Mon, 19 Dec 2011 08:21:53 -0800 > From: noahermesh@gmail.com > Subject: Re: Can I compare two within-subjects' dependent variables while holding co-variates? > To: SPSSX-L@LISTSERV.UGA.EDU > > Hi, > Thanks a lot for your replay. > > My dependent variables are PTSD measures. I have 3 sub-scales and one total > measure. These 4 measures refer to two different events, so each participant > has 8 measures (4 for every event - 4 couples).

Eventually you may do otherwise, but for starters, take *one* measure, like the Total, and see what sense you can make of it. So now you have one measure for two events -- and you seem (below) to be interested in the *difference* of those two. Subtract... and now you have *one* measure for an analysis. If the average is also of interest, take that average, and you have another, single, measure to look at in an analysis.

> > I wanted to compare, for example the degree of avoidance symptoms between > event 1 and 2, and so on, regarding each couple. Each participant has 2 > marks/grads, for each measure - one for every event. > But, than I wand to hold as co-variates the time duration that passed since > each event. I wanted to do that cause there is a significant difference > between the time duration that passed between the two events.

Here's a problem for testing duration since a traumatic event: "duration" is not going to be at all equal-interval or linear, so you will get into trouble if you try to use it as measured merely in days or weeks or months or years. Three years is not much different from 4 years, in respect to recovery, though the difference is 365 days. Ten days versus 375 days is an interval that is much longer, for the difference you would expect.

Maybe you collected the intervals scored into "reasonable" categories. If not, you need to address that problem. I might apply my own "expert" opinion to create categories, if I had some idea. Otherwise, if I didn't want to throw out the detail, I would start by taking the log of the durations, and (possibly) draw in the extremes (Windsordize). For durations, sometimes the reciprocal transformation (events per year, instead of years-per-event) is more appropriate. The reciprocal transformation is stronger that the log, and thus it does more to emphasize differences that are closer to zero.

> While doing an ANOVA or MANOVA, the two co-variates (duration since each > event) influence the two dependent variables, and that is no good. I thinks > there is no point in holding as co-variate the duration since event 1 while > looking at the symptomolgy after event 2. > So, I was wondering how can I compare my dependent variables in a better > way?

But it does make some sense to use both covariates when the criterion is the difference. I think I see a model here with one outcome and two covariates, which is not too hard to perform or interpret.

> > The participants were all volunteers, who agreed to participate in a > web-survey. > > > I hope my answers are clear. > Please let me know if not, and I'll be happy to answer. [snip]

Hope this helps.

-- Rich Ulrich

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