Class 18: Capture Recapture
A common method of estimating the number of fish in a lake or pond is the capture
recapture method. In this method, c fish are caught, tagged, and returned to the
lake. Later on, r fish are caught and checked for tags. Say t of them have tags.
The numbers c, r, and t are used to estimate the fish poplulation.
- 1) What is your estimate for the fish population in terms of c, r, and t. (It may
help to think about actual numbers first.)
- 2) What if some of the tags fall off your fish? Will your estimate be too big or
- 3) Do you think that fish caught the first time are more likely to be caught the
second time (or less likely to be caught)? If so, how would this affect your estimate?
- 4) What other assumptions do you need to make for your estimate to be reasonable?
- 5) The Census Bureau uses capture recapture to assess the number of people who were
not counted by the Census (" the Census undercount"). Can you think how they might
Use capture recapture to estimate the number of Pepperidge Farm goldfish in a package.
So that we can compare results, we ask that everyone capture and tag 50 goldfish
initially, and then recapture 40. Since physically marking or tagging goldfish renders them unappetizing, Laurie prefers to replace the captured goldfish with pizza flavored
goldfish instead. Linda doesn't mind eating magic marker. You can use whatever
method of tagging you like.
Homework for Monday, April 1
Read Chapter 17 and do review exercises 1 - 11.
Journal Assignment for Wednesday, April 3
Read the article "Stalking HIV in the red light district" and the article "High Court
Rules Results are Valid in Census of 1990". Comment in your journal.