Bioequivalence and Bioavailability Forum

Hi David or Guo,

❝ 1. […] please change my username as "Weiguo21224" at your end.

Done.

❝ 2. I carefully reviewed previous discussion. My understanding is that "T" and "R" in FDA code do represent the "Test drug" and "Reference drug" respectively.

Correct so far.

❝ It should be:

❝

❝ ESTIMATE 'T (the large/later one in lexical order, Reference drug) vs. R (the small/early one in lexical order, Test drug') TRT 1 -1/CL ALPHA=0.1;

❝

❝ Right?

In your example and the FDA’s code, yes and no. Alphanumeric codes are internally converted to factors in lexical order and integers by default. At least in all software I know. Examples in :

1. trt <- as.factor(c("T", "R"))            # T > R
print(trt); print(as.integer(trt))
[1] T R
Levels: R T
[1] 2 1
   
2. trt <- as.factor(c("Test", "Reference")) # Test > Reference
print(trt); print(as.integer(trt))
[1] Test      Reference
Levels: Reference Test
[1] 2 1
   
3. trt <- as.factor(c("A", "B"))            # A < B
print(trt); print(as.integer(trt))
[1] A B
Levels: A B
[1] 1 2
   
4. trt <- as.factor(c("tablet", "capsule")) # tablet > capsule
print(trt); print(as.integer(trt))
[1] tablet  capsule
Levels: capsule tablet
[1] 2 1

In all examples – except the third – the levels of the factors are internally reordered. If you are interested in the contrast (difference) of estimated treatment effects, you have to tell the software which of the treatment is the reference. If you don’t, the one with the internally smallest integer will be subtracted from the larger one. Therefore, in the first example you would get R – T instead of the desired T – R.