利用LP法，算TFP，少数值为负，
1. 请问为什么？
2. 如何处理？

命令如下：

levpet lrgyzczxjxgd, free(lcyrs) proxy(lnm) capital(lrgdzchj)

predict tfp_lp1, omega

gen TFP = ln(tfp_lp1)

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