What can we learn from very few data points (with implications for excess death numbers)?

Let's suppose that a museum decides to spend money in Sept 2020 advertising for new members. To see if the advert has worked you manage to find the data for numbers of new members (adjusted for changing population size) in October in each of the 5 previous years. The numbers are:

• Oct 2015: 176

• Oct 2016: 195

• Oct 2017: 169

• Oct 2018: 178

• Oct 2019: 162

Suppose th…