
CaptureTagRecapture
Bud Terfli owns 10 acres of property. He's amazed at
all the butterflies he sees. He wonders...
How many are there? I'd like to know, but I can't
possibly count them all.
Bud consults a statistician, who tells him about a
method that produces an estimate of the population size.
The method is called CaptureTagRecapture.
 Capture
 Bud captures 150 butterflies. He's selected them
using a method called random sampling. That is,
each possible collection of 150 butterflies was
equally likely to have been the captured
collection of 150 butterflies.
 Tag
 Bud tagsor markseach of the 150 butterflies.
Each now has a harmless white spot on its left
wing. Bud releases the butterflies.
 Recapture
 A few days later (the previously captured
butterflies have had ample time to distribute
themselves about the property) Bud again collects
butterfliesthis time he collects a random
sample of 200 butterflies. Some of them have
white spots, some don't. In fact, exactly 17 of
them have a white spot on the left wing.
The statistician provides an onthespot estimate of
the total number of butterflies on Bud's property. Can
you?
 What number does the statistician report to Bud?
Describe the reasoning you use to arrive at this
value.
 Is this number correct? That is, is this number
exactly equal to the true number of butterflies
on Bud's property?
 What practical difficulties arise when using this
method?

