|Date: ||Tue, 12 Aug 2008 12:35:55 -0400|
|Sender: ||"SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>|
|Organization: ||Cornell University|
|Subject: ||interpreting ESTIMATE output GLM|
|Content-Type: ||text/plain; charset=ISO-8859-1; format=flowed|
Suppose I have a single-classification ANOVA, where the sample means
show ordinal trend among successive levels. I construct a CONTRAST to
test for trend, in the usual manner. Fine - but I'm puzzled how to
interpret the parameter output from ESTIMATE (for the same contrast).
For example, consider the following code snippet:
do trt=1 to 5;
do i=1 to 5000;
model x=trt / ss3;
contrast 'linear' trt 2 1 0 -1 -2;
estimate 'linear' trt 2 1 0 -1 -2;
estimate '1 v 2' trt 1 -1 0 0 0;
estimate '2 v 3' trt 0 1 -1 0 0;
estimate '3 v 4' trt 0 0 1 -1 0;
estimate '4 v 5' trt 0 0 0 1 -1;
So, 5 treatment levels, linear increase of the treatment mean by 1 for
each level of trt. So, in fact, the 'slope' of the increase in the
treatment effect is 1 (confirmed by each of the paired estimate
statement contrasting successive treatment means). However, the output
from the linear ESTIMATE is (for a given simulated sample) something like
So, rounding off the estimate to -10 (which is what it should be...
(2)(1)+(1)(2)+(0)(3)+(-1)(4)+(-2)(5) = 10
how do I interpret it? It clearly isn't the slope of the change in
treatment effect (even if I take it and divide it by 5 - the number of
levels of the treatment - it still isn't). While I *know* this is an
acceptable approach to testing significance of a trend, I also need to
estimate the slope of the trend. So, how to go from what ESTIMATE gives
me, to what I actually want/need.
Pointers to the obvious?
Thanks in advance.