A health worker explains that the failure rate of a method is the chance of a pregnancy in one…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A health worker explains that the failure rate of a method is the chance of a pregnancy in one year, and that the chance over many years is larger. She uses two methods from a WHO table of typical use: the male condom, with pregnancies in the first year among users, and the copper releasing IUD, with . She assumes that the chance is the same in every year and that the years are independent of one another.
- What is the chance of at least one pregnancy in years of using the condom?
- What is the chance in years of using the condom?
- What is the chance in years of using the IUD?
- Which method gives the smaller chance in years, and why is the chance after years larger than after years?
Need a reminder? The key ideaShow
A failure rate is the number of pregnancies in the first year among users. Among couples, about pregnancies are expected in a year. The chance of at least one pregnancy in years is . Only the condom, among the methods of this chapter, protects against STIs. Vasectomy and tubectomy are terminal methods.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Condom: . IUD: . Independent years.
Plan. Find the chance of no pregnancy for the years, then take it from .
Step 1. Condom, years. , so , which is .
Step 2. Condom, years. , so , which is .
Step 3. IUD, years. , so , which is .
Step 4. The comparison. The IUD gives the smaller chance, against . A longer time means more years in which a pregnancy can happen, so the chance builds up.
Answer: ; ; ; the IUD, because each extra year adds a chance.
Check. After years the condom chance is about one half, and after years it is about three quarters. It never reaches .
Watch out. The years are not really independent, and users change their habits. The numbers show how a risk builds up, and they do not predict one person's future. This topic is not in the NEET syllabus.
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