Have you ever given a party or planned a wedding and been stressed about ordering or cooking enough food? If you knew exactly how many people were coming, the party would be much less stressful! The problem is that today many people cannot or will not tell you ahead of time if they are coming. People are so last minute in making up their minds. Given this problem, how do you figure out weeks or days ahead of time “how many people are coming to the party” so you can tell the caterer or purchase food for the right number?
I have been using a simple solution for years, that does not involve calling all of my friends and relatives each day to get an update on their status. Instead, I use a key idea from economics and statistics called “expected value.” My wife and I create a very simple list that contains three pieces of information; the guest(s) name, the number of people invited with each guest and the chance that group will come.
We don’t do anything fancy when estimating the chance the guest or group will show up. If they are extremely likely to show up (such as my children) we mark down 100%. If the person is expected to show up most of the time they get a 66%. If the person occasionally shows up they get a 33% and if we would love to see the person but we don’t expect them to show up they get a 0%.
Then we multiply the number of guests in each group by the chance they will show up and add up all the resulting numbers. An example is pictured below. In the example we invited 15 people but expect just 10 on average to show up.
We do the guest list in a spreadsheet so the calculations happen automatically, but the system works even with paper and pencil. By attaching a chance or probability to each guest or group we get the expected number of people in each party. Some of the estimates will be too high. Some of the estimates will be too low. In general the over-estimates cancel out the under-estimates and the resulting number is pretty close to the actual count. The method lowers my stress and reduces leftovers. What could be better?