Posted by
Richard Ristow on
Jan 25, 2014; 4:52pm
URL: http://spssx-discussion.165.s1.nabble.com/studying-different-rates-of-deceleration-tp5724085p5724146.html
At 05:58 PM 1/22/2014, Zdaniuk, Bozena wrote:
>What would be the best way to study different rates of deceleration?
>For example, group A has means for five repeated measurements: 20, 12, 8, 6, 5
>And group B has means: 24, 20, 18, 17, 16
>Both groups show a decelerated curve but group A decelerates at
>greater rate than group B.
At 09:09 AM 1/23/2014, Maguin, Eugene wrote:
>How about fitting a regression with a quadratic term plus a
>group*quadratic term along with the usual group and linear trend terms?
An excellent approach: summarize each time series as three values:
. Mean
. Linear rate of change
. Quadratic rate of change.
For this, you need an independent variable, time, across the repeated
measurements. I'm guessing the measurements are equally spaced in
time, so it's natural to use 1, 2, 3, 4 and 5 as the time values for
the five points.
I'd recommend, instead, centering the time variable; that is using
values -2, -1, 0, 1 and 2. That will dramatically reduce the
covariance of the linear and quadratic trend terms, and give you
cleaner estimates.
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