On 13/06/2021 10:06, Liz Tuddenham wrote:
Rod Speed wrote:

NY wrote
[...]
OK. So this all hinges on the fact that the birth rate (and maybe death
rate) varies throughout the year.

Yes, and all you need is the birth day to vary, the
death day doesn't need to to get the statistical quirk.

That makes sense. Birth rate is under human control (to some extent)
and shows annual peaks, but while death rate only shows general seasonal
trends. If the birth rate is peaky by calendar date and the death rate
is flat, more of the deaths will be on days which coincide with
birthdays - not because there are more deaths on those days but because
there is a greater chance that each dead person had been born on that
day.


Only if expected life is an integral value of years, which there is no
reason to expect it would be.

If expected life were say 82.5 years you would get a peak in deaths half
a year after the peak in births.