Dear stata users and experts,

My data is firm year panel data, i.e., I have observations at firm level and across years. These firms are from different industries identified by industry SIC code.

I have firm specific characteristics, e.g. its inventory turnover ratio (or other financial ratio),

My hypothesis is that one of such ratios could be persistent over time either at firm level or at industry level.

How do I show that the ratios are persistent or it is not?

Let me give you an example, say firm A is in retail industry, its inventory turnover ratio hovers around 10 for the last 14 years (some years slightly higher than 10, others slightly lower than 10), that is quite persistent.

say, the retail industry average inventory turnover is 9 over last 14 years,

given that I have thousands of firms, and over 30 industries, I can't show this by report each firm or industry's mean .

I want to show it in a more efficient way. I know posters here are statisticians, cliometrician, etc. Can you suggest something or a paper I can follow their method ?

Thanks a lot,

Rochelle