how to determine the 3-digit SIC code for industry ?
I'm doing a panel data analyses and I want to control industry effect using industry dummy variable.
to do so, I need to determine two-digit or three-digit SIC code. [as in some articles]
however, the ID number for industries are various with 2, 3, and 4 digit.
[e.g. 21100, 4643, 791 etc]
so how can I convert these different number of digits into only 2 or 3 digit ?