Hi ballardw, Thank you so much for your help! It makes sense. Could I ask one more questions please? I got it that I'd better to use dateofevent and intck('month',dateofevent, "01JAN2016"d) le 6. However, I am not sure how to use the monthly indicator instead of "01JAN2016"d. Below is the further clarification of my data. Actually, in my data, it's not like indicator12_2015 or indicator11_2015. The format is indicator 1- indicator12 for each month (indication 1 is Jan, and indicator 12 is Dec) of each year (dummy variable, 1=yes, and 0=No). In addition, I do have the date of event. My goal is to measure whether the patients had '0' in each monthly indicator within 6 months prior to the date of event. Please let me know if I did not explain it clearly and happy to clarify. Thanks again!
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