interval
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One incident day, plotted as the field. Required staff is the demand curve; effective staff is the schedule curve. Where the two pull apart — a demand surge at 13:00, a collapse at 15:30 — the gap is the instability. Drag shrinkage to see the field deform.
The difference between the staff the workload requires and the staff actually available, after shrinkage. Negative means over-staffed, positive means under-staffed.