Sampling 4 1 Foundations of Sampling Sampling is

Sampling

4. 1 Foundations of Sampling • Sampling is the process of selecting units from a population of interest – Most often people, groups, and organizations, but sometimes texts like diaries, Internet discussion boards and blogs, or even graphic images – By studying the sample, we can generalize results to the population from which the units were chosen

4. 2 Sampling Terminology

4. 2 Sampling Terminology (cont’d. ) • Accessible population – A group you can get access to when sampling, usually contrasted with theoretical population • Bias – A systematic error in an estimate – Can be the result of any factor that leads to an incorrect estimate, and can lead to a result that does not represent the true value in the population

4. 3 External Validity • Generalizing – The process of making an inference that the results observed in a sample would hold in the population of interest – if such an inference or conclusion is valid we can say that it has generalizability • External validity – The degree to which the conclusions in your study would hold for other persons in other places and at other times

4. 3 a Two Major Approaches to External Validity in Sampling: The Sampling Model

4. 3 a Two Major Approaches to External Validity: The Proximal Similarity Model

4. 4 Sampling Methods • Nonprobability sampling – Sampling that does not involve random selection • Probability sampling – Sampling that does involve random selection

4. 5 Nonprobability Sampling • Does not involve random selection – Random selection is a process or procedure that assures that the different units in your population are selected by chance • Two kinds of nonprobability sampling – Accidental – Purposive

4. 5 a Accidental, Haphazard, or Convenience Sampling • Sampling by asking for volunteers • Sampling by using available participants, such as college students • Sampling by interviewing people on the streets • The problem: you do not know if your sample represents the population

4. 5 b Purposive Sampling • Several types: – 4. 5 c Modal Instance Sampling – 4. 5 d Expert Sampling: Validity – 4. 5 e Quota Sampling • Proportional Quota Sampling • Nonproportional Quota Sampling – 4. 5 f Heterogeneity Sampling – 4. 5 g Snowball Sampling: • Respondent Driven Sampling

4. 6 a The Sampling Distribution – Statistical Terms

4. 6 a The Sampling Distribution – A Theoretical Distribution

4. 6 b Sampling Error • Standard Deviation • Standard Error • Sampling Error

4. 6 c The Normal Curve In Sampling

4. 7 Probability Sampling: Procedures • 4. 7 a Definitions: – N is the number of cases in the sampling frame – n is the number of cases in the sample – NCn is the number of combinations (subsets) of n from N – f = n/N is the sampling fraction

4. 7 b Simple Random Sampling

4. 7 c Stratified Random Sampling

4. 7 d Systematic Random Sampling

4. 7 e Cluster (Area) Random Sampling

4. 7 f Multistage Sampling • The combining of several sampling techniques to create a more efficient or effective sample than the use of any one sampling type can achieve on its own

4. 7 g How Big Should the Sample Be? • Bigger is better, as it increases the confidence in results • However, this has to be balanced with cost and time considerations • Sample size is also determined by the sampling technique used

4. 8 Threats to External Validity • Can the results be generalized to other people? • Can the results be generalized to other places? • Can the results be generalized to other time periods?

4. 9 Improving External Validity • Do a good job of drawing a sample from a population – Random selection is always better • Use proximal similarity effectively • Replication – When a study is repeated with a different group of people, in a different place, at a different time, and the results of the study are similar, external validity is increased

Discuss and Debate • What does the term population really mean? What are some examples of populations? • What are the advantages and disadvantages of each sampling technique in this chapter, and when would you choose one technique over another? • What is external validity, and what is the best way to strengthen it?