I have been trying (unsuccessfully) to create a variable that counts upward the number of periods a company is doing business in a particular market, which starts over at 1 when sales fall to $0, or when the market changes for the firm, or when firms change. Any help would be appreciated. The count variable below is what I am shooting for. Firm Market Sales Count 1 100 $354 1 1 100 $344 2 1 100 $244 3 1 100 $37 4 1 100 $0 . 1 200 $98 1 1 200 $99 2 1 200 $0 3 1 200 $34 1 1 200 $25 2 2 100 $0 . 2 100 $34 1 2 100 $44 2 2 100 $39 3 2 100 $0 . 2 200 $98 1 2 200 $99 2 2 200 $23 3 2 200 $34 4 2 200 $25 5 3 200 $9 1 3 200 $9 2 3 200 $2 3 3 200 $3 4 3 200 $2 5
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