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What if preference order changes between raw values and zcd?

In one of my estimations, the order of attribute levels within one attribute (in terms of increasing part-worth utility values) changes when you switch from raw values to zero-centered differences. What does that mean for interpretation?

In general: How would you interpret the values of zero-centered differences (in contrast to raw utilities)?
asked Feb 14, 2017 by summand (385 points)
retagged Feb 14, 2017 by Walter Williams

1 Answer

+1 vote
One of the things to remember about part-worth utilities is that they are estimates, not exact "true" points.  There is a range of uncertainty surrounding each one.  We can compute a standard error (the standard deviation divided by the square root of the sample size) and then multiply the standard error by +/- 1.96 to obtain a 95% confidence interval.  We are 95% confident that the true utility falls within that range.  

My guess is the change in preference order you saw for levels within an attribute when comparing zero-centered diffs to raw utilities did not represent a statistically meaningful change in preference.  But, you could compute the 95% confidence intervals for either (or both) of the raw utilities or the zero-centered diffs to see if the 95% confidence intervals overlapped for those two levels that traded places.

Zero-centered diffs does a better job more equally weighting respondents than raw utilities, where some respondents (the most clean and certain ones) can have a larger weight in influencing the mean utilities.

The interpretation of the utilities follows the same rules.  Zero-centered diffs are just placed on a bigger scale.  The overall magnitude of the scale does not change your interpretation of the part-worth utilities.  You could multiply all the part-worth utilities by 2 or 100 or 1000 and your conclusions should not change.
answered Feb 15, 2017 by Bryan Orme Platinum Sawtooth Software, Inc. (132,290 points)