I am dropping observations based on various if statements written in a do file. I have all the code in the do file highlighted and run it. All I do is "clear" and "log close" in the command window, and hit run again without touching the highlighted code or even reselecting the code. I've run the code 10 times in a row this way, and sometimes I get n=1620 and sometimes I get n=1621. No code has changed between each time I run it, so I am positive that Stata is producing different results when given the exact same input. Why is this happening?