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We have 10 participants who have undergone a cold pressor test (for pain) in 3 conditions (no drug, drug A, drug B). It is a repeated measure design. The stimulus is continued for 1 minute after they start reporting pain. The length of time before they report pain is their 'tolerance / threshold'. Each participant, in each session, gets the stimulus (i.e., 1 minute of cold after they start reporting pain, i.e., after the pain threshold has been reached). The pain score reported on VAS (a continuous scale) is the main outcome. However their tolerance / threshold is a covariate (their pain scores may be related to their tolerance times which would otherwise not be controlled for).

How should this data be dealt with? Should I analyse the threshold and pain report (VAS) as 2 separate 'independent' outcomes? Or should I stick with pain report as the main outcome (and use Pain threshold as a covariate? If the latter, how do I do it? Using RMANOVA in SPSS?

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  • $\begingroup$ Thank you for your replies and useful advice. I hope someone can advise me on how to use covariance in RMANOVA? I can see a covariant option in SPSS when entering data (dependents and factors)..... however, since each drug session has its threshold time (so 3 covariates, 1 for each drug session) .....how are those managed in SPSS ? Thanks again $\endgroup$
    – user59327
    Commented Oct 25, 2014 at 23:11
  • $\begingroup$ Please register & merge your accounts (see here). Then you will be able to comment on your own question & on answers to it. Note that we are not an SPSS tech support site, just a statistics Q&A site. There is information on where to get SPSS-specific help on the internet here. $\endgroup$
    – gung - Reinstate Monica
    Commented Oct 25, 2014 at 23:39
  • $\begingroup$ How much is the correlation between pain threshold and pain report (VAS)? $\endgroup$
    – rnso
    Commented Oct 26, 2014 at 3:25

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No advice on doing this in SPSS, but it appears to me that the pain threshold is in the causal path, I.e., it is affected by the drug.

If so, analysis of covariance techniques whereby you compare adjusted means of VAS -- predictions of VAS at the same pain threshold -- could be very misleading because you wouldn't expect the same mean pain threshold with each drug.

Continuing with the idea that pain threshold is in the causal path... One option is to simply ignore the pain threshold data. But the downside is that it could account for a lot of variation in the response variable, so the power of your tests are reduced.

Another option is to adjust the covariate for treatment effects before fitting the model, then use the adjusted covariate instead of the original one. The ANCOVA textbook by Rutherford (publ by Wiley) describes this technique. Then you get the reduction of error variance without obtaining misleading predictions. The basic technique is to fit the treatment model with pain threshold as the response, then the adjusted pain threshold values are the residuals from this model.

I don't use SPSS a so I can't tell you how to fit a covariance model with repeated measures, but it ought to be possible.

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  • $\begingroup$ This is a really good point. What would you think of using rmMANOVA w/ threshold & pain score both as response variables? $\endgroup$
    – gung - Reinstate Monica
    Commented Oct 25, 2014 at 17:11
  • $\begingroup$ I think the multivariate model would be appropriate too, but the interpretation/followup is less straightforward, and it (sort of) treats the two measures equally even though VAS is the primary response measure. $\endgroup$
    – Russ Lenth
    Commented Oct 25, 2014 at 17:53

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