Estimating Population Size by Random Sampling
Teacher Notes
Time needed to complete lab:
A 40 minute class period
Target Grade level:
It should work well for younger Middle School students. For older students it
could be the first exercise in a series that leads to knowledge of basic statistics. Students
interested in ecology will find it useful in understanding sampling design of wild
populations.
Questions:
1. Your boss tells you that the percent error is too large. You have to go out to sample
the population again. What can you do this time to reduce the percent error?
Explain your answer.
The sampling design was truly random; it cannot be improved. The only way to reduce
the percent error is to take a larger number of samples. When only a few quadrants are
sampled, chance may lead to an inaccurate estimation of population size. As in any
sampling design, the more random samples that are taken, the closer the population
estimate will be to the actual population.
2. Let us say that you sample the ten grid quadrants and get an estimated population, but
You can not determine the true population size. Therefore, you can not calculate the
percent error. What can you do to reduce the percent error, even though you do not?
Know what it is. Hint: would the answer you gave for question 1 also apply here?
Explain.
Yes, the answer is the same as the answer to question 1. Whether you know the actual
population size or not, taking more samples will give estimates that are closer to that
size.
3. It takes a lot of time to draw all of those slips. Why not take a pencil and circle 10 or
so quadrants and count the spiders or rats in them? Would this give about the same?
Percent error as the method you used? Explain.
Any quick-and-dirty method of sampling is subjective and will usually give unreliable
estimates of population size. The method of randomly drawing slips forces one to take
truly random, unbiased samples from the grid.
There are 141 spiders in the grid, or 1.41 spiders per grid segment.
There are 134 rats in the grid, or 1.34 rats per grid segment.
Population Estimation
Rev. 6/27/2008
Page | 1