Calendar and population adjustments

Today let's understand why a raw monthly total mixes the calendar, the size of a population and genuine growth together, and how to tell them apart.

Victoria's monthly turnover for cafes, restaurants and takeaway food services is a real series the Australian Bureau of Statistics has tracked every month since April 1982. Over 441 months it climbs from \$81.3 million a month to \$1,066.2 million a month.

Here is the whole series, plotted in the order the months actually happened.

Look at that climb for a second. It is not a clean, steady line: it fluctuates up and down within every single year, and near the end that yearly swing is far bigger than the one back in 1982. Some of that climb, and some of that swing, is the calendar and the size of the country talking, not real growth.

Same month, different lengths: how many days each month has

Start with the simplest reason a raw monthly total can move: some months are just longer than others. February usually has 28 days. March, right next to it, has 31. That is three extra days for turnover to be recorded on, before any real change in how busy the shop actually is.

Look at Victoria's cafe, restaurant and takeaway turnover for every month of 2018, next to how many days each of those months actually held.

December, at \$1,066.2 million, is both this series' single highest month across the whole 1982 to 2018 history and a 31-day month. February, at \$810.6 million, is 2018's smallest month, and it is also the shortest, 28 days. Some of December's size and some of February's smallness is simply that one of them had three more days to sell things on than the other.

That does not mean day count explains the whole gap between them. But it means a raw total cannot be read on its own: some of the difference between two months is genuine, and some of it is just how many days each month happened to hold.