sample definitions sample types rules for simple random sample and sample size selection

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