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Colin Johnson’s blog


Puzzle (1)

Here is a graph that purports to be a summary of numbers of divorces per 1000 married people between 2009-2016, i.e. the first part of the graph up to 2014 is before same-sex marriage became legal.

divorces per 1000 married people

My immediate thought is that this must be wrong—if every marriage is between a man and a woman, then the numbers of divorces must be equal between men and women. So, could the “per 1000 married people” be the gotcha here? Again, no. It doesn’t say “per 1000 people”, but “per thousand married people”, and so in the era that this is referring to, the number of married men and married women would be identical. This suggests that there is an error in the calculation here; oddly, the graph has identical numbers from 2013 onwards; we might expect some divergence if we carry on with the graph, even simply due to statistical fluctuations the number of same-sex male divorces and same-sex female divorces is likely to be different.

So, what is happening during the 2009-2012 part of the graph? I suspected initially that they have mistakenly used “per 1000 people” on those entries in the graph, rather than “per thousand married people”. But, this is at odds with the numbers from 2013-2016, where the graph is as expected—numbers “per thousand people” will be a lot less than “per thousand married people”, and this huge leap isn’t apparent between the figures for 2012 and 2013. So, what explains it?

I’ll restrain myself from ranting about the heinous sin of connecting discrete values with lines.

Here’s another graph (from this Daily Mail article (ugh!)) that seems to be from the same source and shows a similar error:

another similar graph

One Response to “Puzzle (1)”

  1. colingj says:

    Ooh, there’s an explanation on the ONS website. “The male divorce rate is calculated by dividing the number of males divorcing in a particular year by the estimated number of married males aged 16 and over in that same year (taken from mid-year population estimates by marital status). Likewise, the female divorce rate is calculated by dividing the number of females divorcing in a particular year by the estimated number of married females aged 16 and over in that same year. While the actual number of males and females getting divorced in a particular year is equal, the number of married males does not equal the number of married females because 1 partner could live away, either overseas or in a communal establishment such as a care home or prison. For this reason, divorce rates for males and females can differ for a particular year.”

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