请问各位高手，SAS proc genmod中的offset命令的意义，什么情况下要应用这一命令呢？我查了些相关资料，有的是下面这样子说的，但我看不明白是怎么回事，望高手指点啊！
The most widely used regression model for multivariate count
data is the log-linear model (see McCullagh and Nelder, 1989):
l
og(E(Yi)) = log ti + β ′ xi
where β is a vector of regression coefficients, xi is a vector of
covariates for subject i, so called offset variable log ti is needed
to account for possible different observation periods (ti) for
different subjects.