So I am currently trying to write a paper estimating the Constant Elasticity of Substitution Production Function of the USA. I am using the simple version with two inputs capital and labour. Since the estimation is at the aggregate level I thought of using the perpetual inventory method to create the data for aggregate capital, but I wasnt sure what kind of data I need for aggregate labour. Maybe the number of employed people, work force, in the USA? I'd be glad if someone could help me out.