Chapter3.34

# Chapter3.34 - 3.3 Sampling Designs The population is the...

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3.3 Sampling Designs The population is the entire group of individuals or objects about which we want information. The information collected is constrained in a sample which is the part of the population we actually get to observe. The idea here is that we wish to study some attribute in a population. For example, the proportion of Minnesota voters that will vote for Current Governor Tim Pawlenty in the next election. The population of Minnesota is too large to examine each voter, but we can learn a lot by selecting a subset of voters, study them, and then extrapolate our findings from the small subset to the entire state. The key is to take a representative sample, or subset. That is, a good sample. It is good to use plans or designs for doing the sampling. Here are a few designs that might be used. How the sample is chosen, that is, the design, has a large impact on the usefulness of the data. A useful sample will be representative of the population and will help answer our questions.

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“Good” methods of collecting a sample include the following: Simple random sample , also called SRS Probability samples Stratified random samples Multistage samples All these sampling methods involve some aspect of randomness through the use of a formal chance mechanism. Random selection is just one precaution that a person can take to reduce bias , the systematic favoring of a certain outcome. Simple Random Sampl e This is the most straightforward design. A simple random sample of size n consists of n individuals from the population chosen in such a way that every set of n individuals has an equal chance to be the sample actually selected . If we wished to survey the UMM student population we would generate a list of all registered students. Then from this list we would use a random number generator on a computer or a table of random numbers to select say n =75 students randomly from the master list.
Probability Sample A probability sample is a sample chosen by chance. We must know what samples are possible and what chance, or probability, each possible sample has. SRS

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Chapter3.34 - 3.3 Sampling Designs The population is the...

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