Yes, giving birth twice in 24 months is normal, but that gives you a very narrow window within 2018/2019.
If you give birth in July of 2018, an 18 month spacing won't let you give birth in any month of 2019. That's still a very significant negative adjustment to "demographic factors get you to 5X+". Demographic factors aren't the only factors there are.
This is the birth rate (for 2016, in the US) reflecting the odds that a randomly-selected person, male, female, infant, or menopausal, will give birth in a calendar year.
> Fertility rate: 62.0 births per 1,000 women aged 15-44
And this is the birth rate demographically adjusted for the ability to give birth. It's about five times higher.
Since siblings born in consecutive calendar years are rare, we can only conclude that patio11's "true statement" is false by a pretty large margin. For it to be true, every birth in 2016 would have to have been to a woman who also gave birth in 2015.
I wish that, before asserting something is true, people would think about whether it's true.
> we can only conclude that patio11's "true statement" is false by a pretty large margin. For it to be true, every birth in 2016 would have to have been to a woman who also gave birth in 2015.
So this has been bothering me, because if every birth in one year is to a woman who gave birth the previous year, and the years have equal birth rates, and we ignore aging in/out of the "fertile" demographic, then the odds of giving birth in one year given that you gave birth last year are 100%, well over 5x the population rate.
The claim is that the odds of giving birth this year given you gave birth last year are equal to the odds of giving birth this year given you're fertile, or in other words that, if you're fertile, whether you give birth this year is probabilistically independent of whether you gave birth last year. This is quite clearly false -- your odds of giving birth in one year are much lower, given you gave birth the year before, than your odds of giving birth in any randomly-selected year during which you're fertile. But my argument above is wrong.
For patio11's claim to be true, 6.1% of women who gave birth in one year would need to give birth again the next year. (Stated equivalently, 6.1% of people would need to have an older sibling born one calendar year before themselves.)
Good call with that sanity check. Patio11's assertion definitely had that paradoxical lightbulb moment that makes you want to believe it is true (a la Malcolm Gladwell), but two data points were all that was necessary to expose it.
If you give birth in July of 2018, an 18 month spacing won't let you give birth in any month of 2019. That's still a very significant negative adjustment to "demographic factors get you to 5X+". Demographic factors aren't the only factors there are.
edit: a quick sanity check tells us the factor of 5 estimate is off by a huge amount. From https://www.cdc.gov/nchs/fastats/births.htm :
> Birth rate: 12.2 per 1,000 population
This is the birth rate (for 2016, in the US) reflecting the odds that a randomly-selected person, male, female, infant, or menopausal, will give birth in a calendar year.
> Fertility rate: 62.0 births per 1,000 women aged 15-44
And this is the birth rate demographically adjusted for the ability to give birth. It's about five times higher.
Since siblings born in consecutive calendar years are rare, we can only conclude that patio11's "true statement" is false by a pretty large margin. For it to be true, every birth in 2016 would have to have been to a woman who also gave birth in 2015.
I wish that, before asserting something is true, people would think about whether it's true.