What do statisticians use to plan wedding guest lists?
Statistical modelling, and luck:
Vukcevic placed his guests into four categories, ranked in order of likely attendance, from “definitely” to “unlikely”. He assigned each category a probability and added some further assumptions: families would either attend all together or not at all; beyond that, one guest’s decision to attend would be uncorrelated with another guest’s. With a target of 100-110 attendees, and an absolute maximum of 120, Vukcevic and Ko celebrated with … 105 people.
[However] Vukcevic’s assumptions were flat-out wrong. For each of the four categories, the actual attendance rates were lower than forecast. (“Likely” attendees had an assumed attendance rate of 80 per cent and an actual attendance rate of zero.) Vukcevic predicted that the chance of having 100 or more invited guests attend was more than 99 per cent; only 97 did, a result which the model said was vastly unlikely.
… And yet Vukcevic and Ko celebrated with “an ideal number” of attendees. How so? Another assumption proved felicitously flawed: that the chance of uninvited guests was zero. Happily for Vukcevic and Ko, their failure to account for wedding crashers cancelled out all the other mistakes.