Levinsohn-Petrin TFP estimation in Stata without proxy

I'm trying to compute TFP by using levpet Stata command. I use revenue estimation, yet it requires proxy option which is intermediate input (usually material resources used for goods/services production (as far as I understand)). Unfortunately, I don't have this data as is in my table. And other authors when referring to usage of levpet write that they use hours worked and capital only. The question is whether there is a way in Stata to leave proxy blank (by dummy column or something like that) or may be I can use one of the columns I have as in intermediate input. I have the following fields in the table: various types of liabilities, depts, ebitda, operating expenses, sales, cost goods sold, profits, investment cash flow, various assets, operating revenue turnover, revenue. I thought may be it has sense to use cost of goods sold or investments as intermediate input for the proxy option of levpet.

• Without claiming any expert knowledge of the topic, I believe the whole point of the LP methodology is to avoid using investment as a proxy given its limitations. So what is the rationale for using investment with the levpet command? Wouldn't that defeat the purpose of LP? Not that using investment as a proxy is wrong, but sounds like that's not LP. The Stata Journal paper that introduced that command is Production function estimation in Stata using inputs to control for unobservables, Petrin et al (2004), freely available with a Web search. Oct 20, 2014 at 2:54
• Hmm, good point. I don't use Stata. The estimation is so similar when using investment and using materials as a proxy that I just assumed levpet would support both. It in fact does, but in a limited way. The command levpet output, free(labor materials) capital(capital) proxy(investment) valueadded will use capital and investment as the control, and treat labor and materials as flexible. If instead, you want to treat labor as predetermined like capital, then it should be included in the control as well. This option doesn't seem to be available.