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Expected attendance is confirmed yeses plus roughly a third of non-responders, reduced by the historical no-show rate. Catering is then ordered slightly above that figure, because running out of food is a far worse outcome than a little waste.
Attendance estimate
expected = (yes + 0.35 x non-responders) x (1 - no-show rate); cater for expected x 1.05
5-10% for a paid event with committed guests, 20-30% for free events, and higher still for casual invitations.
Non-response correlates with disorganisation rather than disinterest. The exact fraction varies by event type, so track your own history.