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Re: T-test???
Posted:
Jun 12, 1996 1:24 PM
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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 T-test
at a 95% confidence level. I don't think the T-test 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 t-test is also fairly robust to deviations from the
assumptions of normal distribution. So the t-test is slightly wrong,
but not likely to yield wildly misleading conclusions.
Somewhat better alternatives are offered by nonparametric tests such
as Wilcoxon two-sample. 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
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