Estimate the number of weddings per year - approach problems

Question

Today, my case partner asked me to estimate the number of weddings per year in a given country. Let's arrive to the core of the case in simplified terms. After dividing the population in four age sections, we came up with this situation:

0-20: 0 weddings

21-40: 200 weddings

41-60: 100 weddings

61-80: 20 weddings

Now, in this given population we have 320 potential weddings. I was going to divide 320 weddings by 60 years, but my case partner stopped me and told me that I had to divide by 20 years (200/20 + 100/20 + 20/20 or 320/20).

The two approach are not equivalent, hence one is wrong. Which one and why?

PS: If dividing by 20 years is correct, I have another doubt. Let's say that we approach the case differently and we segment the population in only one big age cluster (21-80). Now, in order to have the same number of weddings per year (320/20 = 16), the number of potential wedding in the same population should be 16*60 = 960. For this reason, I suspect that dividing by 20 is not the correct option.

Many thanks for the effort!

Answer 1

Tbh I am not convinced this approach works at all - at least it is very unintuitive. It starts with the number of weddings per age bracket. The way you set up the calculation (deviding by size of the bracket later on), this seems to be an absolute number, not one per time interval (e.g. per year).

But this doesn't make sense, because an absolute definition doesn't seem to make sense. The population in each age bracket changes year on year, so whatever you're calculating is at least highly unintuitive.

In addition, splitting this into age groups introduces unnecessary complication, as you don't care when people marry, only if they marry throughout their lifetime.

A much simpler approach could be to calculate annual birth rate (e.g. total population devided by life expectancy). Then you can estimate the number of weddings per person (probably slightly above 1). Multiplying the number of births per year with the number of weddings in a life time and devided by 2 (takes two persons to marry once), should give you the number of weddings per year.