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•In proportionate stratified sampling, each stratum’s size is proportionate to the stratum’s share of the population. •Any stratification that departs from the proportionate relationship is disproportionate. ADVANTAGES = Control of sample size in strata, Increased statistical efficiency, Provides data to represent and analyze subgroups, Enables use of different methods in strataDISADVANTAGES = Increased error if subgroups are selected at different rates, Especially expensive if strata on population must be created, High costiv. Cluster sampling
Similar to stratified samplingInvolves dividing population into subgroups (i.e. clusters)Involves random sampling of the clustersand all members of the subgroupare sampledUseful when the population is divided into mutually exclusive subgroups (or clusters), e.g.SuburbsSchoolsSlabs of beers on a production linePlanes arriving at Newcastle AirportPopulation divided into heterogeneous subgroups. Some are randomly selected for further study. Use when there is a need for more economic efficiency than can be provided by simple random sampling; and Unavailability of a practical sampling frame for individual elements.Several questions must be answered when designing cluster samples.How homogeneous are the resulting clusters?Shall we seek equal-sized or unequal-sized clusters?How large a cluster shall we take?Shall we use a single-stage or multistage cluster?How large a sample is needed?
ADVANTAGES = Provides an unbiased estimate of population parameters if properly done, Economically more efficient than simple random, Lowest cost per sample, Easy to do without listDISADVANTAGES = Often lower statistical efficiency due to subgroups being homogeneous ratherthan heterogeneous, Moderate cost- Clusters can be areas – area sampling : well defined political or geographical boundaries, low cost, frequently used- Stratified vs Cluster sampleVS- Summary of probability sampling methodsStratifiedPopulation divided into few subgroupsHomogeneitywithin subgroupsHeterogeneitybetween subgroupsChoice of ClusterPopulation dividedinto many subgroupsHeterogeneitywithin subgroupsHomogeneity between subgroupsRandom choice ofsubgroups
v. Double sampling •In drawing a sample with double (sequential or multiphase) sampling, dataare collected using a previously defined technique. •Based on the information found, a subsample is selected for further study.ADVANTAGES = May reduce costs if first stage results in enough data to stratify or cluster the populationDISADVANTAGES = Increased costs if indiscriminately usedb. Nonprobability samples•Subjective approach: Probability of selecting population elements is unknown. •Greater opportunity for bias in the sample and distorted findings.