One thing you can do is use tracked data in the summary sheet.
Each time someone does not keep an appointment, you go to Summary Sheet --> Tracked Data --> Add/Edit (date is automatic) enter DKNA in your data field and "positive" without the quotes in the value field.
If you run reports from the main window and use:
Category = Tracked Data
Field = Tracked Data Value
Operator = equal (=)
Value = positive
Run the report and everyone put in the tracked date value, will show up in your query.
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As an fyi, you can click on Did Not Keep Appoint (red x) under their name in Patient List. Send to the doctor. Then the doctor notes it and saves it to chart. You now have a record in Past Encounters.