In reply to Leonid Batkhan.

Sure!
This is the code that I am running:

data data2;
merge data1 data1(firstobs=2 rename=(Date1=lead1_Date));
run;

Where "data1" consists of an ID variable and 'Date1'.
In "data1", the first ID has 3 observations and when I perform the data step above, they are only showing 2 observations, though the total number of observations in the "data2" matches that of "data1". I'm not sure how. I'm sure I'm missing something very fundamental here haha.