topic Re: How do I interpret the log transformed CL for the difference in SAS GLM compared to normal scale in Statistical Procedures

How do I interpret the log transformed CL for the difference in SAS GLM compared to normal scale

jacksonan123 — Wed, 07 Sep 2016 16:05:54 GMT

ods trace on /listing;
ods output estimates=stat1;
ods output FITSTATISTICS=stat2; 
ods output LSMEANCL=stat3;
ods output OverallANOVA=stat4;
ods output LsmeandiffCL=stat5;
ods output LsmeanCL=stat6;
proc glm data=new1;
class trt;
model clnr=trt/ss1 ss2 ss3 ss4;
lsmeans trt/pdiff cl alpha=0.10;
estimate 'A vs B' trt 1 -1;
quit; run;

ods trace on /listing

ods output estimates=stat7;
ods output FITSTATISTICS=stat8; 
ods output LSMEANCL=stat9;
ods output OverallANOVA=stat10;
ods output LsmeandiffCL=stat11;
ods output LsmeanCL=stat12;

proc glm data=newL1; 
class trt; 
model Lclnr=trt/ss1 ss2 ss3 ss4; 
lsmeans trt/pdiff cl alpha=0.10; 
estimate 'A vs B' trt 1 -1; quit;

The code was used to analyze the attached data set CLnr1. The outputs from stat6 and stat12 were compared which are the Lsmean CI.

Stat6

Obs Effect Dependent TRT LowerCL LSMean UpperCL 1 2

TRT	CLnr	A	102.984141	114.620000	126.255859
TRT	CLnr	B	38.763546	47.962500	57.161454

Stat12

Obs Effect Dependent TRT LowerCL LSMean UpperCL 1 2

TRT	LCLnr	A	4.554136	4.732812	4.911488
TRT	LCLnr	B	3.699795	3.841051	3.982307

The values of exp(lower CL) and exp(upper CL) give values for A (94-134) and B(40-53) which are close to the values for the normal scale.

However for stat5 for the difference on the normal scale I get:

bs Effect Dependent i j LowerCL Difference UpperCL TRT _TRT 1

TRT

CLnr

51.824632

66.657500

81.490368

Whereas for stat 11 for the LClnr I get

s Effect Dependent i j LowerCL Difference UpperCL TRT _TRT 1

TRT

LCLnr

0.663993

0.891761

1.119529

If I do the exp(lowerCL) and exp(UpperCL) I get (1.93-3.03) which is not in agreement with the normal scale.

Can someone explain to me why the normal scale and log scale agree for the CL for A and B but not for the difference between A and B for the normal and log scale?

Re: How do I interpret the log transformed CL for the difference in SAS GLM compared to normal scale

PGStats — Wed, 07 Sep 2016 17:24:05 GMT

On the log scale, Difference is an estimate of log(A) - log(B) = log(A/B)

So, EXP(Difference) is an estimate of the ratio A/B, not A - B.

Re: How do I interpret the log transformed CL for the difference in SAS GLM compared to normal scale

jacksonan123 — Wed, 07 Sep 2016 17:51:03 GMT

I have used EXP(difference) which based upon your response is not correct.

Therefore my question is how should the log(A/B) ratio on the log scale be exponentiated to have it reflect the A-B difference on the Normal scale for the respective confidence intervals?

Re: How do I interpret the log transformed CL for the difference in SAS GLM compared to normal scale

Doc_Duke — Thu, 08 Sep 2016 22:36:10 GMT

When you take a log transform, you are now in the realm of the geometric distribution. Google

geometric mean site:sas.com

for lots of information on your question.

Re: How do I interpret the log transformed CL for the difference in SAS GLM compared to normal scale

SteveDenham — Mon, 12 Sep 2016 17:26:04 GMT

This is why it is a good idea to use GENMOD or GLIMMIX, so that results can be presented on both the transformed and original scale. PROC GLM just can't do that.

Steve Denham

Re: How do I interpret the log transformed CL for the difference in SAS GLM compared to normal scale

jacksonan123 — Mon, 12 Sep 2016 17:53:10 GMT

I was able to get the CI on the log scale (see code below) by use of the command LRCI. How do I get the CI

m on the normal scale per your suggestion?

Proc genmod data=y;

class trt;

model lauc144=trt/lrci;

run;

Re: How do I interpret the log transformed CL for the difference in SAS GLM compared to normal scale

jacksonan123 — Tue, 13 Sep 2016 10:38:04 GMT

Thisresolved the issue.

Thanks