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Re: Ttest???
Posted:
Jun 12, 1996 1:24 PM


In article <4pgk6g$prc@mochi.lava.net> rfujita@lava.net writes:
After running several experiments, I realized that I needed to classify my outcomes SUBJECTIVELY as unfavorable, slightly unfavorable, neutral, slightly favorable or favorable. Although using these results isn't very ideal, time constraints leave very little alternatives. What I need to do is to determine the expected outcome of an event based only on the number of experiment I ran and the subjective rating I gave it.
I was thinking of converting the ratings to a number scale (i.e. unfavorabe=0, slightly unfav=1, ..., favorable=4) and running a Ttest at a 95% confidence level. I don't think the Ttest is appropriate for subjective "measurements" though.
As you note, the choice of scores is arbitrary and hence a change in mean score is difficult to interpret. On the other hand if the general differences you wish to find are tendencies for the distribution to move in a particular direction, the means *should* differ. The ttest is also fairly robust to deviations from the assumptions of normal distribution. So the ttest is slightly wrong, but not likely to yield wildly misleading conclusions.
Somewhat better alternatives are offered by nonparametric tests such as Wilcoxon twosample. These have the feature of being independent of the assigned scores. Also, the rank sums that enter have a rather nice interpretation, being related to the number of times an observation from group 1 is bigger that one from group 2 (sum over all pairs).
Make sure you use the Wilcoxon test corrected for *ties* (with only 5 categories, you are bound to get a lot of observations with the same value and you have to use average ranks).
 O__  Peter Dalgaard Blegdamsvej 3 c/ /'_  Dept. of Biostatistics 2200 Cph. N (*) \(*)  University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~  (p.dalgaard@biostat.ku.dk) FAX: (+45) 35327907



