I have panel data on firms in 3 countries (e.g., 100 firms in 3 countries in 5 years, and event happen at the 3rd year in all countries at the same time). For an example, for each firms I have 4 variables, including:

y x1 x2 profitability

(while y is dependent variable and the rest are independent variables). I am examining the impact of a law on y by using differences-in-differences. There is a theory that suggests that the law has more impact on firms' with high profitability. Therefore, I want to test the impact of laws on a sample of firms with high profitability. However, each firms has 5 profiatbility (given my sample period is 5). So is there any conventional way or widely-used approach to getting a high-profitability subsample in such a case?


1 Answer 1


There isn't any single way of how to do it. A reasonable way is to calculate average profitability over these 5 periods, ordering firms from most to least profitable and then making the sub-sample from the top 20% or 25% or $x%$ depending on what you believe is reasonable.

  • $\begingroup$ This make senses somehow to me, I am wondering if is there any reference to it? $\endgroup$ Jan 9 at 17:41
  • 1
    $\begingroup$ @PhilNguyen no reference, it is just a reasonable thing to do. $\endgroup$
    – WilliamT
    Jan 9 at 17:42

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