I attached a sample for the dataset. The sample include two variables: personid and ym. Personid refer to the specific ID for each trader and ym refer to the year-month variable. For each personid, I want to estimate the number of years over the 10 years period prior to month t. For example, a person trade in 200001, 200301, 200501, 200801, and 201501. Then, for 200301, the number of year is 1. For 200801, the number of year is 3. For 201501, the number of year is 2. If the trading year is continuous, I assume I could use by function to calculate the difference between the first trading year and the current trading year and the difference will be the number of year. However, the trading year for each trader is not always continuous. In this case, I do not know how to count the number of observation. Please give me some suggestions. Thanks.
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