Unit 10 - Introduction to sampling Flashcards
what is the definition of population?
Entire body of units of interest to the decision maker
what is the definition of sampling?
Selection of a part of the population for the ultimate purpose of drawing general conclusions about the entire population
what is the definition of sampling frame?
a list of all the items in your population. It’s a complete list of everyone or everything you want to study.
What are the steps to the sampling process?
- Define the population
- Identify the sampling frame
- Develop a sampling plan
- Decide on sample size
- Do the actual selection
what is the definition of element?
the object about which or from which the information is desired
what is the definition of a sampling unit?
A sampling unit is an element, or a unit containing the element, that is available for selection at some stage of the sampling process
what does extent refer to?
the geographic boundaries
what are the types of probability sampling?
- Simple Random Sampling
- Systematic Sampling
- Stratified Sampling
- Cluster Sampling
what are the types of non-probability sampling?
1 Convenience Sampling: Used to obtain information quickly and inexpensively
2 Judgmental Sampling: “Expert” uses judgment to identify representative samples
3 Snowball Sampling:
• Form of judgmental sampling
• Appropriate when reaching small, specialized populations
• Each respondent, after being interviewed, is asked to identify one or more others in the appropriate group
4 Quota Sampling:
• Minimum number from each specified subgroup in the population
• Often based on demographic data
What factors correspond to nonprobability sampling?
- Exploratory research
- Errors are larger
- Population is homogeneous
Statistical considerations: Unfavourable
Operational considerations: Favourable
What factors correspond to probability sampling?
Conclusive research Sampling errors are larger Heterogeneous population Statistical considerations: Favorable Operational considerations: Unfavorable
Comment on probability sampling
All population members have a known probability of being in the sample
Comment on Simple Random Sampling
Each population member, and each possible sample, has equal probability of being selected
Comment on Stratified Sampling
The chosen sample is forced to contain units from each of the segments or strata of the population
What are the types of stratified sampling?
- Proportionate Stratified Sampling: Number of objects/sampling units chosen from each group is proportional to number in population
- Disproportionate Stratified Sampling: Sample size in each group is not proportional to the respective group sizes
what are the characteristics of cluster sampling?
- Involves dividing population into subgroups
- Random sample of subgroups/clusters is selected and all members of subgroups are interviewed
- Very cost effective
- Useful when subgroups can be identified that are representative of entire population
what are the strengths and weaknesses of simple random sampling (srs)?
strengths: Easily understood; results projectable
weaknesses: Difficult to construct sampling frame; expensive; lower precision; no assurance of representativeness
what are the strengths and weaknesses of systematic sampling?
strengths: Can increase representativeness; Easier to implement; Sampling frame not needed
weaknesses: Can decrease representativeness
what are the strengths and weaknesses of stratified sampling?
strengths: Includes all important subpopulations; precision
weaknesses: Difficult to select relevant stratification variables; not feasible to stratify on many variables; expensive
what are the strengths and weaknesses of cluster sampling?
strengths: easy to implement; cost effective
weaknesses: Imprecise difficult to compute and interpret results
how may quota sampling be viewed?
Quota sampling may be viewed as two-stage restricted judgmental sampling.
•The first stage consists of developing control categories, or quotas, of population elements.
•In the second stage, sample elements are selected based on convenience or judgment.
what are the approaches to decide on a sample size?
- Unaided judgment: arbitrary approach
- All-you-can-afford: cost consideration
- Average size for sample for similar studies
- Required size per cell
- Bayesian statistical method
- Traditional statistical method: the main focus
what information is needed to calculate sample size?
- Specification of the error (e) that can be allowed.
2. Confidence coefficient
how to select fieldworkers?
The researcher should:
•Develop job specifications for the project, taking into account the mode of data collection.
•Decide what characteristics the field workers should have.
•Recruit appropriate individuals.
Do you need to train the fieldworkers?
Interviewers should be trained to:
make opening remarks that will convince potential respondents that their participation is important.
• Asking the Questions:
- Be thoroughly familiar with the questionnaire.
- Ask the questions in the order in which they appear in the questionnaire.
- Use the exact wording given in the questionnaire.
- Read each question slowly.
- Repeat questions that are not understood.
- Ask every applicable question.
- Follow instructions, skip patterns, probe carefully.
how to evaluate the fieldworker?
Cost and Time. The interviewers can be compared in terms of the total cost (salary and expenses) per completed interview.
Response Rates. It is important to monitor response rates on a timely basis so that corrective action can be taken if these rates are too low.
Quality of Interviewing. To evaluate interviewers, the supervisor must directly observe the interviewing process.
Quality of Data. The completed questionnaires of each interviewer should be evaluated for the quality of data.
