topic Re: How to get the change rate using proc mixed? how to explain proc mixed output? in SAS Programming

How to get the change rate using proc mixed? how to explain proc mixed output?

PSZ — Wed, 28 Nov 2018 23:04:15 GMT

I have 1000 persons data with 5 years' annual bone mineral density (BMD) measurement. Now I want to get the bone mineral density (BMD) change rate per year. Because this is longitudinal data, I think I should use proc mixed.

My data is like:

id visit group bmd

1 0(baseline) a 1.01

1 1 a 1.00

1 2 a 1.00

1 3 a 0.99

1 4 a 0.98

1 5 a 0.96

2 0(baseline) b 1.07

2 1 b 1.06

2 2 b 1.04

2 3 b 0.99

2 4 b 0.95

2 5 b 0.93

......

My code is :

PROC MIXED DATA=mydaa noclprint;
CLASS id visit group;
MODEL bmd = visit group group*visit / SOLUTION CL DDFM=KR OUTPM=aaa;
REPEATED visit / SUBJECT=id TYPE=UN R RCORR;
RUN;

Is this code correct? and where can I get the change rate in the output? I read one paper said the estimate is the change rate, but the numbers are positive (the change rate should negative).

And I plot the result like this:

PROC SGPLOT DATA=aaa;
SERIES x = visit y = pred / GROUP = group MARKERS;
RUN;

The plot looks correct. But how can I get the numbers for the slop?

Thank you very much for your help.

Re: How to get the change rate using proc mixed? how to explain proc mixed output?

rbikes — Mon, 10 Dec 2018 18:42:54 GMT

The estimate is only the estimate of the dependent variable. Here, the dependent variable is bmd, is bmd a change rate? Looking at the structure I would say it isn't. Assuming I am correct I would suggest you create baseline variable and then from that and bmd calculate the change and use the change variable as the dependent variable and also include baseline into the model.

Re: How to get the change rate using proc mixed? how to explain proc mixed output?

PSZ — Mon, 07 Jan 2019 22:38:52 GMT

Thanks for your reply. If I understand correctly, the coefficient of the interaction term (group * visit) (i.e. the slop of the model) is the change rate.

Re: How to get the change rate using proc mixed? how to explain proc mixed output?

rbikes — Mon, 07 Jan 2019 23:45:12 GMT

If you just want to find the slop I would suggest to follow this tutorial by Rick Wicklin

https://blogs.sas.com/content/iml/2013/02/27/slope-of-a-regression-line.html