Study Population and Sampling Design ⭐️

Entire group from whom information will be obtained and to whom results are generalized

What is a population?

List of all possible members of the population eligible for sampling

What is a sampling frame?

A subset of the population used for sampling

What is a sampling frame in relation to population?

Individual member of the population that can provide data

What is a sampling unit?

Selected sampling units chosen using proper methods

What is a sample?

Should ideally match population characteristics but may not 100%

What is the ideal relationship between sampling frame and population?

Official databases like household lists or student enrollment lists

What are common examples of sampling frames?

Calculated number of sampling units selected for the study

What is sample size?

Actual sources of data for research

What are samples?

Receive survey forms or data collection instruments

What happens to samples in a study?

Each unit has a known and equal chance of selection

What is probability sampling?

Units do not have a known and equal chance of selection

What is non-probability sampling?

Simple Random Sampling, Systematic, Stratified, Cluster, Multistage Sampling

What are examples of probability sampling methods?

Assigns equal chances to each sampling unit, traditionally by random draw or computer-generated numbers

What is simple random sampling?

Randomizes only the starting point, then selects others at fixed intervals

What is systematic sampling?

Divides population into groups (strata), samples randomly within each group

What is stratified sampling?

Groups are heterogeneous with homogeneous sampling units (e.g., males vs females)

What characterizes strata in stratified sampling?

Divides population into groups (clusters), randomly samples entire clusters

What is cluster sampling?

Microcosms with heterogeneous units but similar distribution across clusters

What characterizes clusters in cluster sampling?

Requires statistical adjustments for undercoverage bias

What is a limitation of cluster sampling?

Includes the entire population (census)

What is total enumeration sampling?

Convenience, Quota, Judgmental (Purposive) sampling

What are common non-probability sampling methods?

Selects easily available units; purposive selects specific units deliberately

How do convenience and purposive sampling differ?

Often biased and non-generalizable but useful in qualitative or constrained studies

When is non-probability sampling appropriate?

Should fit research objectives and practical constraints

What should guide the choice of sampling method?

Limitations of sampling strategy should be disclosed in manuscripts

What should researchers disclose about their sampling methods?

To avoid bias and ensure ethical and valid results

Why do we define sample size before starting a study?

Too few subjects prevent generalization and detection of differences, risking unethical study

What happens if the sample size is too small?

Too many subjects risk unnecessary exposure to intervention and waste resources

What happens if the sample size is too large?

Calculation of optimum participants needed for valid results

What is the purpose of calculating sample size?

Acceptable level of significance, power, expected effect size, event rate, and standard deviation

What factors affect sample size?

Expected drop-out rate, unequal allocation, study objective and design

What other factors may affect sample size?

“p” value indicating probability result is due to chance (commonly p < 0.05 or p < 0.01)

What is the level of significance?

Probability of correctly detecting a difference (1 - β), usually set at 80%

What is the power of a study?

Difference between control and test groups, expressed absolutely or relatively

What is expected effect size?

Large effect size requires smaller sample; small effect size requires larger sample

How does effect size influence sample size?

(Prevalence) estimated from prior studies for sample calculation

What is the underlying event rate?

Measures data variability; larger variability requires larger sample size

What is standard deviation in sample size calculation?

Homogeneous populations with small variance need smaller samples

How does population homogeneity affect sample size?