Module 4 - Session 4.5a Non - Probability Sampling Methods

1. Module 4 - Session 4.5aNon - Probability Sampling Methods

2. Probability sampling Random Systemic Cluster Stratified Spatial Complex Common Sampling Designs

3. Probability sampling Random Systemic Cluster Stratified Spatial Non-probability sampling Convenience Snowball Purposive Key informants Focus groups Quota Common Sampling Designs

4. Non-probability Sampling Overview Recruit people that are easy to reach or belong to a group that the assessor is interested People could be best/worst/typical/unique case Criteria must be established on how people are recruited and how characteristics are assessed Recruitment must be systemically selected Primarily used for pilot testing

5. Non-probability Sampling Issues Only report on people that were interviewed People interviewed may not be representative of the target population Cannot make statistically valid generalizations about target population Can make statements about the target population Range of answers Inventory outcomes Presence of a condition

6. Convenience Sampling Overview Draw sample from people that are available and easy to reach Able to get range, inventory, or presence of a condition Used for pilot testing such as a random survey Low cost and quick results May match and compare convenience samples to reduce bias

7. Snowball Sampling Overview Use when sample characteristic is rare Study subjects recruit other subjects from people they know Often used in hidden populations – commercial sex workers, iv drug users, IDPs, nomads People who have many friends are more likely to be recruited Friends tend to have similar characteristics

8. Purposive Sampling Overview Sampling with “a purpose” in mind Assessor recruits people who fit their criteria Best/worse/typical case Hard to reach groups – nomads, IDPs Live in certain area Able to quickly reach targeted sample when sampling for representativeness is not a major concern Usually excludes unique or special cases Often used in rapid assessments

9. Key Informants Sampling Overview People with local knowledge of the target area and event causing crisis No structured interview Typically reconnaissance in conjunction with other surveys Identify presence or range of a characteristic such as a health clinic or distance to a market Information often influences future surveys and sampling efforts

10. Focus Groups Sampling Overview Combine purposive sampling and self-selection Usually ask 6-12 people their attitudes, behaviors, and opinions Questions are asked in an interactive group setting where participants are free to talk Can be used to gain access to cultural and social groups Results have high validity Low cost, fast results Use – assess wants and needs of clients, monitor and evaluate services, look at underlying causes of food insecurity

11. Focus Group Sampling Issues Assessor has less control over group than in a one-on-one interview Small size, inexpensive Recruitment may be difficult Need experienced observers or moderators Members of focus group not large enough to be representative sample of a population Assessor may influence results Assessor are participants of focus group Focus groups tend to please rather than offer their own opinions Focus more on community than individual attitudes, behavior and opinion

12. Quota Sampling Overview Non-probability equivalent of stratified sampling Assessor identifies stratums and their proportions Convenience or purposive sampling used to recruit respondents Select the first people who fit criteria regardless of what group they represent Key is to select people who match target population on certain characteristics (age, sex, etc).

13. Quota Sampling Issues Selection based upon judgment of interviewer Only a few characteristics can be simultaneously considered No call backs for not-at-home or refusal-conversions to complete interviews Data “looks good” enough to be representative but there is no randomization and therefore no idea of the chance of being selected Data may be “generalizeable” if quota sample characteristics match target population characteristics

14. a warning… You may know that results of a survey using non-probability sampling should not be generalized to a larger population but the reader of your report may not. Please consider footnoting this phrase in a report that uses non-probability sampling: “This survey used non-probability sampling to select participants. The results should not be used to make generalizations about a larger population because the respondents were not randomly selected and may not be representative of the larger population. In addition, the chance of being selected for this survey is unknown and other people in the population being studied had no chance or an unknown chance of being selected for this survey.”