Biased and Unbiased Surveys
Date: 11/05/2008 at 17:34:48 From: Caitlin Subject: 'bias in surveys' What does 'biased' and 'unbiased' mean? Here's an example. Tell weather the sampling method is biased or unbiased: The Tri-State Soccer League is conducting a survey to determine if the players want to change the style of soccer shirt. They will randomly survey all players who wear size large shirts. I don't know what 'biased' and 'unbiased' mean.
Date: 11/07/2008 at 01:33:50 From: Doctor Jordan Subject: Re: 'bias in surveys' Hi Caitlin, The basic idea of statistics is to make statements about a set of things using a subset (a sample) of those things. A sample is "biased" if some members of the population are more likely to be included than others. A sample is "unbiased" if all members of the population are equally likely to be included. Here are two examples. Suppose I want to find out how big a typical fish is in a lake. One way of getting a sample of the fish would be to use a net. But then I will never catch any of the fish that are smaller than the holes in the net, so I'll think that all the fish in the lake are big. This sample is biased because the big fish are more likely to be included in my sample; to get an unbiased sample I need to sample the fish in a different way. Now suppose I want to know how long a bear sleeps every day on average during the year. If I watched it for 10 days in the winter, each day it would sleep 24 hours since bears hibernate in the winter. This sample is biased because in winter the bear sleeps all day while during the rest of the year it is awake part of the day and asleep part of the day. If I want an unbiased sample, I could watch it for 1 day each month for a year, instead of just in the winter. In your example about the Tri-State Soccer League, there are a lot of players and we need to take a sample of them to find out if they want to change the shirts. If we only sample players who wear large shirts, this sample is biased because some members (those who wear large shirts) are more likely to be included. The problem with this is that maybe the medium shirts are too big but the small shirts are too small for some players, while the large shirts fit well. If we only sample players who wear large shirts, we will not find out about the fitting problems with the other sized shirts. Does this make sense? If you have any questions please write back. - Doctor Jordan, The Math Forum http://mathforum.org/dr.math/
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