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Unformatted text preview: rates, of students in large stat class are probably representative pulse rates of all college‐aged people. Sample versus Census Why can’t we learn about a population by just taking a census (measure every item in the population)? Takes too long, costs too much, measuring destroys the item. So, we often rely on a special type of statistical study called a sample survey, in which a subgroup of a large population is questioned on a set of topics. Sample surveys are often used to estimate the proportion or percentage of people who have a certain trait or opinion. If you use proper methods to sample 1500 people from a population of many millions, you can almost certainly gauge the percentage of the entire population who have a certain trait or opinion to within 3%. The tricky part is that you have to use a proper sampling method. 24 Bias: How Surveys Can Go Wrong While it is unlikely that the sample value will equal the population value exactly, we do want our surveys to be unbiased. Results based on a survey are biased if the method used to obtain those results would consistently produce values that are either too high or too low. Selection bias occurs if the m...
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This document was uploaded on 02/25/2014 for the course STATS 250 at University of Michigan.
 Summer '10
 Gunderson
 Statistics

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