## The Problem With Simple Statements

This quote is taken from the Guardian: 'In 2015 teenage pregnancy rates fell to an all times low in the UK - 21 conceptions per thousand teenage girls aged under 18'. Good news maybe. Consider the meaning of the statement.
Does it mean that 21 girls under 18 were pregnant? No. Some girls would have had abortions or miscarried, or otherwise not have had the baby. A full term pregnancy only lasts 9 months if it goes to term, so about
$\frac{9}[12} \times 2.1=1.575 \percent$
of girls under 18 are pregnant at any one time, ignoring other considerations.
Does it mean that if you randomly pick a girl under 18, there is a probability of 21/1,000 that she will be pregnant? No. Apart from the point above, some girls become serially pregnant, and serially miscarry or have abortion. For the same reasons, the probability that a randomly selected girl has been pregnant in the past year is 1,1,000 will fail.
Does it mean that 21 out of every 1,000 girls became pregnant before they are 18? No. The quote refers to a window of a year - 2015. In that year, 21 out of every 1,000 girls under 18 became pregnant.
The phrase 'under 18' is jut a bit misleading. A girl aged 2 will not be pregnant, so the pregnancy rate among girls aged 13 - 17 should be higher.