Category: statistics

Friday, 24 August 2012

07:58 – Big article on the front page of the paper this morning about that moron politician’s comments about women who are raped being unlikely to get pregnant. The article throws lots of numbers around. The problem is, all of those numbers are so soft that they’re pretty much useless. No one knows how many forcible rapes actually occur, or even how to define the term “forcible rape”. No one knows how many pregnancies result. And the one number all of the parties seem to take as a given seems ridiculously high to me: that a woman who has sexual intercourse, forced or voluntary, has a 5% chance of becoming pregnant.

So I did a little thought experiment. The population of Winston-Salem is roughly a quarter million, 50% of whom are female, or roughly 125,000. Assume that one third of those women are sexually active, or roughly 42,000, and assume that each of them has sexual intercourse once a week. Obviously, the frequency varies. Women in their 20’s may average having intercourse several times a week, ones in their 30’s once or twice a week, ones in their 40’s once a month, and so on. But once a week on average seems reasonable. That means sexual intercourse occurs in Winston-Salem 42,000 times a week, or 6,000 times a day. By the assumptions made in the article, 300 babies a day would be conceived in Winston-Salem.

Obviously, that’s not the case, and it’s because the assumptions made in that article are simplistic. First, most women in their prime child-bearing years use birth control, often pills or a patch, so they’re very unlikely to be impregnated during intercourse, forced or voluntary. Second, the age of the woman is a huge factor. Women’s fertility peaks in the decade between their late teens and late 20’s, and then begins to decline rapidly. Third, of course, although most pregnant women continue having intercourse during their pregnancies, any woman who is already pregnant has zero chance of being impregnated.

It makes a lot more sense to look at the big picture. US women, on average, bear about 2 children over the whole course of their child-bearing years. Although there are obviously exceptions at both ends, call that ages 14 to 44. During that time, these women have sexual intercourse hundreds to thousands of times. Again, call it an average of once a week, or about 1,500 times over those 30 years. So, 1,500 intercourses result in two pregnancies, or a rate of 2/1,500 or about 0.133%. Not 5%.

Read the comments: 48 Comments
// ------------------------------------------------------------------------------- // end of file archive.php // -------------------------------------------------------------------------------