Lecture 13 9/19/24 Flashcards
What are the potential sources for data collection?
-total population/census
-designed/formal sample
-non-designed/informal sample
What are the advantages and disadvantages of collecting data from a total population?
advantages:
-true picture
disadvantages:
-too expensive
-time consuming
-not always practical
What are the advantages and disadvantages of collecting data from a sample?
advantages:
-less costly
-saves time
-more practical
disadvantages:
-may not truly represent population
What are primary data samples?
information collected directly by an investigator for their specific study, such as through survey or observational study
What are secondary data samples?
samples are taken from existing data or previously collected by someone else
What are the general considerations for sampling?
-objectives of the study
-target population
-what to measure/how much to measure
-precision
-sampling units
-variables
-population characteristics
-practicality
-cost and time
What are the criteria for the acceptability of sampling method?
-each individual unit has equal chance of selection
-representative of the population sampled
-measurable validity
-feasible
-economical and efficient
What are the characteristics of non-probability sampling?
-no scientific/explicit approach
-mostly used in analytical studies
-selection bias likely but often unknown
-no mathematical property
What are the types of non-probability sampling?
-judgement: experts select units to include
-convenience: units that are easy to obtain are included
-purposive: units are chosen based on some attributes
What are the characteristics of probability sampling?
-scientific approach
-used in descriptive and analytical studies
What is the required information for probability sampling?
-sampling fraction: proportion of population to be included in sample
-sampling frame: list of all sampling units in source population
-sampling unit: basic elements of the population that is sampled
What is simple random sampling?
-every individual in the source population has equal probability of being included
-complete list of the sampling frame
What are the advantages and disadvantages of simple random sampling?
advantages:
-equal probability of selection
-able to calculate confidence intervals
disadvantages:
-bias if population is not homogenous
-difficult if sampling frame cannot be defined
What are the characteristics of stratified random sampling?
-used when population is not homogenous for characteristics of interest
-ensures that each group in the population is represented
What is the procedure for stratified random sampling?
-identify sampling frame
-sort into strata
-select a simple random sample within each stratum
Why is stratified random sampling used?
-provides proportional representation to every strata
-ensures that individuals from smaller strata of the population are included in sufficient numbers
When can stratification occur?
-during sampling
-during analysis
What are the advantages and disadvantages of stratified random sampling?
advantages:
-more flexible than SRS
-improves precision
disadvantages:
-must know which stratum each subject belongs to prior to sampling
What are the characteristics of systematic random sampling?
-also known as interval sampling
-fixed percentage of the population is selected at equal intervals
-complete list of sampling frame is not necessary
What is the procedure for systematic random sampling?
-sampling interval j is calculated as the study population size divided by the required sample size
-first unit is randomly chosen, and every jth study subject after that is included in the sample
What are the advantages and disadvantages of systematic random sampling?
advantages:
-easy and practical
-does not require knowledge of total population size or sampling frame
-ensures that sampling units are distributed evenly over entire pop.
disadvantages:
-characteristics being estimated may be related to interval itself
-difficult to assess variability of estimates
When is cluster sampling used?
-when identification of the sampling frame is impractical and/or too costly
-when observation units are aggregated into larger sampling units
What is the procedure for cluster sampling?
take a simple random sample of the population of clusters, which is used to estimate the measure of all observational units within each cluster
What are the advantages and disadvantages of cluster sampling?
advantages:
-practical
-cheaper than simple random sampling
-flexible
-works great in homogenous pop.
disadvantages:
-requires larger sample size to obtain same precision as simple random sampling
What are the characteristics of multi-stage sampling?
-sampling occurs are multiple stages
-different sampling methods can be used at different stages
What are the advantages and disadvantages of multi-stage sampling?
advantages:
-practical
-cheaper
-flexible
disadvantages:
-requires larger sample size to obtain same precision as simple random sampling
