Sample DefinitionsSample Types

Rules for Simple Random Sampleand

Sample Size Selection

Basic Terms• Sampling• Element• Population• Sample• Sampling Frame• Representative Sample v. Bias SampleRules 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