Week 4 (Chapters 8 & 9)

6. What qualitative factors should be considered in determining the sample size?

1. the importance of the decision 2. the nature of the research 3. the number of variables 4. the nature of the analysis 5. sample sizes used in similar studies 6. incidence rates 7. completion rates; and 8. resource constraints

1. What is the main difference between a sample and a census?

A census is a complete enumeration of the elements of a population. A sample is a subgroup of the population selected for participation in the study.

14. Describe stratified sampling? What are the criteria for the selection of stratification sampling?

It is a probability sampling technique that uses a two-step process to partition the population into subpopulations or strata. Elements are selected from each stratum by a random procedure , usually SRS.

11. What is the relationship between quota sampling and judgmental sampling?

Judgemental sampling is a form of convenience sampling in which population elements are selected on the researcher's judgment. Quota sampling is a non-probability sampling technique consisting of two-stage restricted judgmental sampling.

10. What is the main difference between judgmental and convenience sampling?

Judgmental sampling is a form of convenience sampling in which the population elements are selected based on the judgment of the researcher. The researcher, exercising judgment or expertise, chooses the elements to be included in the sample, because he or she believes that they are representative of the population of interest or are otherwise appropriate. In convenience sampling the

7. What are incidence rates? How do they affect the sample size?

They are the rate of occurrence or the percentage of persons eligible to participate in the study. They determine how many contact need to be screened for a given sample size requirement.

13. Describe the procedure for selecting a systematic random sample (SRS)?

1. Define the population 2. Choose your sample size 3. List the population 4. Assign numbers to cases 5. Calculate the sampling fraction 6. Select the first unit 7. Select your sample

3. Describe the sampling design process.

1. Define the target population 2. Determine the sampling frame. 3. Select sampling technique(s) 4. Determine the sample size 5. Execute the sampling process

2. Under what conditions would a sample be preferable to a census? A census preferable to a sample?

A sample is preferred when there are time limits and budget restraints. A census is preferred when the population size is small and there is a large variance in the characteristics.

5. What is a sampling unit? How is it different from the population element?

A sampling unit is the basic unit that is available for selection at some stage of the sampling process.

17. What factors should be considered in choosing between probability and non-probability sampling?

Budget and time constraints. Non-probability sampling is less expensive but can be time consuming. In probability sampling, it can also be cost effective but it can decrease representativeness and may have low precision.

9. What is the least expensive and least time consuming of all sampling techniques? What are the main limitations of this technique?

Convenience sampling. The limitations are that there are many potential sources of selection bias present including respondent self-selection. Convenience samples are not recommended for descriptive or causal research, but they can be used in exploratory research for generating ideas, insight or hypotheses.

12. What are the distinguishing features of simple random sampling?

In SRS, each element of the population has a known and equal probability of selection. Furthermore each possible sample of given size, has a known and equal probability of being the sample actually selected. So every element is selected independently of every other element. The sample is then drawn by a random procedure from a sampling frame.

16. Describe the cluster sampling procedure. What is the key distinction between cluster sampling and stratified sampling?

In cluster sampling, all the individuals are taken from randomly selected clusters. In stratified sampling, randomly selected individuals are taken from all the strata.

8. How do probability sampling techniques differ from non-probability sampling techniques?

In probability sampling, each element of the population is selected by chance. In non-probability sampling the researcher decides what elements to include in the sample.

15. What are the differences between proportionate and disproportionate stratified sampling?

In proportionate stratified sampling, the size of the sample drawn from each stratum is proportionate to the relative size of the stratum in the total population. Disproportionate sampling requires that some estimate of the relative variation, or standard deviation of the distribution of the characteristic of interest, within strata be known. As this information is not always available, the researcher may have to rely on intuition and logic to determine the sample size of each stratum.

4. How should the target population be defined?

The target population needs to be defined precisely. Imprecise definition of the target population will result in research that is ineffective at best and misleading at worst. Defining the target population involves translating the problem definition into a precise statement of who should and should not be included in the sample.