The sum of coefficient are 0 using STATA program?
Generate log versions of the output variable and the three inputs. For example:
gen lvadd=log(vadd) and do the same for pay, invest and material.
Run a regression of lvadd on the three log input variables, and year. Test the hypotheses that:
(5 points each)
3) the sum of the input coefficients=1. You may either do this with a by-hand F-test or use STATA's test command (which is itself an F-test), and it's even possible to do this with a t-test, if you really want to try. Rejection implies either increasing or decreasing returns to scale. Failure to reject means constant returns to scale.