I often need to get the CI for the ratio of means for bioequivalence testing. I know using a log transformation on the data and then converting back the associated CIs accomplishes this. The problem I have is the data contains negative values (% change in lesion counts). I have tried adding a constant, but the resulting CIs change depending on the constant I use. I am using the following code to convert the CI of the difference of the treatment means into a CI for the ratio of means:
*getconfidence interval for difference of treatment means;
odslisting close;
odsoutput lsmeandiffcl=cla(rename=(dependent=_name_));
procglm data=pp(where=(extrt ne 'Vehicle'));
class extrt siteid;
model pchinf=extrt siteid;
lsmeans extrt/out=lsm alpha=0.10stderr pdiff cl; *get lsmeans for treatment;
run;
quit;
odslisting;
dataequci&outvar;
merge cla lsm(where=(extrt='Reference'));*usereference lsmean for denominator;
by _name_;
length ciles $30;
*reverse sign because above CI calculatedon reference-test difference instead of test-reference difference;
ciles='('||cats(put((-uppercl/lsmean)+1,5.3))||','||cats(put((-lowercl/lsmean)+1,5.3))||')^{super1}';
run;
I believe the method is sound, but I am unable to find any kind of documentation to back it up.
Any comments?
Craig