Thursday, August 2, 2012

Sponges Worthy or IUD and Diff-in-Diff

On my subway ride today I read an article on technology used in Seinfield episodes that we no longer use (movie phone, answering machines) and of course contraceptive sponges are no longer the method of today. Which is too bad because Avinash Dixit has created a model which helps show who is sponge worthy.

A good article in Slate helps us see the latest in birth control, particularly related to teens. The article first talks about falling teen birth rates and concludes that abstinence only education vs receiving information on birth control is not likely the cause. The author uses a simple method economists often refer to as difference in difference (diff-in-diff or DID for short). Suppose we see that Maryland had a teen pregnancy rate of 8% in 2000 and after 10 years of a new sex ed program that started that year we seen the pregnancy rate is 5%. We don’t know if the program is the cause in the reduction of teen births perhaps other factors. What we can do is compare Maryland to a similar state to one with similar other trends that had abstinence only. If we see in the other state that the teen pregnancy rate also fell 3 percentage points it is more likely that other factors besides the type of sex ed are at play. The article argues that all states have seen similar declines in teen pregnancy regardless of education. So using diff-in-diff we see no impact of sex ed.

The real change seems to be the new IUDs, that prevent pregnancy better than the pill for the average teen (since only 1 in 4 teens remember to take the pill each day and 10% of teens on the pill became pregnant in one year according to one study).

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