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Sample Definitions Sample Types Rules for Simple Random Sample and Sample Size Selection. Basic Terms. Sampling Element Population Sample Sampling Frame Representative Sample v. Bias Sample Rules for getting a good representative sample
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Sample DefinitionsSample TypesRules for Simple Random Sampleand Sample Size Selection
Basic Terms • Sampling • Element • Population • Sample • Sampling Frame • Representative Sample v. Bias Sample Rules for getting a good representative sample • Every element has an equal chance of being selected. • No element can be selected more than once.
Types of Sampling • Probability • Non-probability
Probability Sampling • Simple Random Sample (SRS) • Systematic Random Sample • Stratified Random Sample • (Proportional Stratified Random Sample) • Cluster (Random) Sample
Non-Probability Sampling • Availability Sample • Purposeful/Judgmental • Snowball • Quota • Informant • Focus Group • Criteria
Rules for a Collecting a Simple Random Sample (SRS) • Every subject within a population must have an equal chance of being selected. • No subject within a population may be selected more than once. Note: Violation of either one of these rules can produce false generalizations. In addition, compliance with these rules is very expensive. For example, the standard fee to conduct a national phone survey is $1,500 per minute. Because of this expense, social service agencies rarely collect adequate samples. Therefore their results cannot be trusted.
Sample Size Selection • The problem with the following formula: • It is celebrated for dichotomous data. The sample size will increase with the number of options given to the subject.
One More Rule There are many formulas for determining a sample size. Regardless of what formula you use, the rule of thumb is, “increase your sample size by 20%.”
Check out the following • http://stattrek.com/Lesson3/SamplingTheory.aspx?Tutorial=Stat’ • http://www.youtube.com/watch?v=Qo2K46Hr3lc • http://www.youtube.com/watch?v=g9H_XGqmvEs