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Parmesan or Pretzel? WSS Helps Demonstrate ‘Counting America’

13 May 2010 923 views No Comment
Ameliz Vogel, U.S. Bureau of Labor Statistics

February 27 marked the second annual African–American History Program Free Family Day at the Marian Koshland Science Museum in Washington, DC. This year’s theme was “Counting America,” an event about the history and the perceptions surrounding the decennial U.S. Census. In addition to the Washington Statistical Society (WSS), the program included representatives from the Bureau of the Census, the National Archives, and the American Anthropological Association.

To illustrate the census theme, the group demonstration featured a capture-recapture study using a large bowl filled with Pepperidge Farm goldfish crackers, to simulate a lake populated with fish. Pretending to be wildlife ecologists, the presenters caught a sample of the goldfish population, tagged them, and released them back into the wild of the fishbowl. During the presentation the captured pale yellow, Parmesan-flavored goldfish were replaced with an equal number of dark brown pretzel goldfish—in effect “tagging and releasing” the first sample.

A second sample was selected from the fishbowl, and the proportion was determined of tagged pretzel fish that were recaptured. Since the total number of pretzel fish in the population was known (the representatives made sure to count their first tagged sample), the presenters were able to estimate the total population size contained within the fishbowl from the proportion of pretzel fish that were caught in their second sample. By accumulating the results through repeated sampling, with the assistance of museum visitors, the estimate became a better and better approximation of the true population size of all fish in the pond—Parmesan and pretzel.

How does this demonstration relate to the census theme of the event? Capture-recapture methods are also used to estimate homeless populations. An investigator may visit a location and interview homeless people present on the day of the visit. By returning to that same location on subsequent visits and noting the proportion of the same individuals who were interviewed during the first visit, the total homeless population for that location can be estimated.

Koshland Museum’s Family Day was a smashing success, with about 150 visitors in attendance, from elementary school children to adults. The WSS representatives were asked wonderful questions and the demonstration engaged the visitors. Of course, offering small paper cups with extra “nonexperimental” goldfish for snacking may have helped in attracting an audience. To view the web announcement about the event, click here.

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