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Random Sampling

Random Sampling. Is that Random?. Sample. A s ample is a collection of data from some fraction of a population. It allows us to learn about the entire population by studying a relatively small portion of the population .

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Random Sampling

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  1. Random Sampling Is that Random?

  2. Sample • A sample is a collection of data from some fraction of a population. • It allows us to learn about the entire population by studying a relatively small portion of the population. • A sample frame is the collection of individuals from which a sample is drawn.

  3. Census • A census is when every individual in the population is measured. • Not difficult if your population is Mr. Rick’s 7th period. • Imagine your population is all household in the Commonwealth of Virginia. • What would happen by the them you collected data from the entire population? • Would there be some households that could be easily missed?

  4. Census vs. Sample • Suppose you are interested in knowing the mean height for everyone in your school. • What would a census tell you? • How close would a sample size of one less student that the population be to the mean of the population? • What about ten less students? • If you took a sample of 100 and another sample of 10, which mean would be closer to the population?

  5. Census vs. Sample • In general, a larger sample size will result in a more accurate estimate of the population. • When the population is very large, larger samples will not produce significantly better results and are usually more expensive or difficult to obtain. • A balance is usually made, not too small to sacrifice accuracy, and not too large to sacrifice cost. • For very large populations a sample size in the hundreds is normally sufficient.

  6. Random • Making sure that a sample is truly random means that the person taking the sample has no control over who is selected. • When a sample is not random a bias could appear. This means that some individuals are somehow favored over others in the population.

  7. Simple Random Sample • A Simple Random Sample (SRS) is the result of a sampling method that assures every possible sample of the same size has an equal probability of being selected. • Describe a method that would ensure your sample of 5 students from this class will be a simple random sample. Compare and discuss your method with others in the class.

  8. Simple Random Sample • Random Numbers are a primary tool in selecting SRS. • You can get random numbers from your TI-84 calculator. • Books of random numbers have been published. Page 834 • Websites exist which provide truly random numbers. http://www.random.org/

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