Posted on Fri 12 September 2008

Bristol vs Sarah

Now that we now that Sarah Palin didn’t fake her pregnancy and Bristol Palin is carrying Obama’s baby let’s talk about something else,

Assume that:

A = Has Down syndrome
B = Mother is between 40-44 years old
C = Mother is between 15-19 years old
Total Births = 4,265,996
15-19 year olds gave birth to 435,427 babies
40-44 year olds of age gave birth to 105,476 babies
Probability of having a Down’s child if 17 is 1 in 1200=0.000833333333
Probability of having a Down’s child if 44 is 1 in 30=0.0333333333

For Sarah
P(A|B)P(B)/P(A) = 0.033 * (105,476/4,265,996) / 0.00125 = 0.65

For Bristol
P(A|C)P(C)/P(A) = 0.00083 * (435,427/4,265,996) / 0.00125 = 0.068

It was never likely that Bristol gave birth to a child with Down syndrome, even if births among girls her age are more than four times as common as women of Sarah Palin’s age.

I guess the moral of the story is that while certainly not optimal, and not something I recommend, it is better to have a baby at 17 than at 44.

[Washington Post (pdf)]

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