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PHC 6000: Sampling

Introduction to epidemiology: Sampling

QuestionAnswer
What are probability samples? Any method of sampling that utilizes some form of random selection
What are non-probability samples? Methods of sampling that does not involve random selection
What are the four possible combinations of exposure/outcome that participants in a cross-sectional study can have? -Exposure & outcome -Exposure & no outcome -No exposure & outcome -No exposure & no outcome
When should nonparametric methods of analysis be used? When sample is not a random or probability sample or when a sample doesn't fit a particular distribution
What is essential to generalize to the source population? Representativeness (validity)
What is a sampling unit (element) Subject under observation on which information is collected
What is an element also called? Sampling unit
What is a sampling frame? List of all the sampling units from which the sample is drawn
What is a sampling scheme? Method of selecting sampling units from sampling frame
What is a sampling fraction? Ratio between sample size and population size
What is probability sampling? Each member of the population has a known chance (probability) of being included in sample
When are nonprobability samples useful? When probability samples are not feasible
What are some situations in which probability samples may not be feasible? Groups that cannot be penetrated/hard to reach (e.g., prostitutes, prescription drug users)
What is one of the main problems with nonprobability samples? They are less generalizable
Sampling error is easily measured using what type of sampling methods? Simple random sampling and systematic random sampling
What are the disadvantages to simple random sampling? -Need complete list of units -Doesn't always achieve best representativeness -Units may be scattered/poorly accessible
What are the disadvantages to systematic sampling? -Need complete list of units -Periodicity
What are the advantages to stratified sampling? -Can acquire info about whole population and individual strata -Precision increased if variability within strata is less than between strata = homogenous
Give an example of variability within strata being less than between strata. There is less variability within females than between males and females
What are the disadvantages to stratified sampling? -Can be difficult to identify strata -Loss of precision if there are small numbers in individual strata
What is a solution to small numbers in individual strata? Sample according to proportion of stratum population; weight according to size
What are the advantages to cluster sampling? -Provides logistical advantage -Complete list of sampling units within population not required -Less travel/resources required
What are the disadvantages to cluster sampling? Potential problem is that cluster members are more likely to be alike than those in another cluster
What needs to be considered when choosing a sampling method? -Population to be studied -Size/geographical distribution -Heterogeneity with respect to variables -Availability of list of sampling units -Level of precision required -Resources available -Data collection method (e.g., phone, interview)
Which variable should be used to determine sample size? Choose the variable that would require the largest sample size
Created by: AlneciaPHS