For Randomized Control Trial (RCT) to assess dating in adolescent, a study was done by selecting random schools, then random classes, then random sections and then random students. This is an example of:
Correct Answer: Multistage sampling
Description: Ans. d. Multistage sampling (Ref: BK Mahajan 6/e p100-101; Park 23/e p849, 20/e p752-753)In Multistage sampling, sampling procedure is carried out in several stages using random sampling techniques.Multistage sampling is used when the population units arranged in hierarchal groups (Sampling schools, then sections, then students).In a sampling technique, every 10th unit of population is chosen. This is type of systematic random sampling.Types of Sampling Random SamplingNon-random samplingSynonyms* Probability samplingQ* Non-purposive sampling* Non -probability sampling* Purposive samplingQTypes* Simple random sampling* Systematic random sampling* Stratified random sampling* Multistage random sampling* Multiphase random sampling* Cluster random sampling* Convenience sampling* Quota sampling* Snow-ball sampling* Clinical trial samplingTypes of Random SamplingSimple Random Sampling:Every unit of population has equal and known chance of being selectedQIs also knowm as 'unrestricted random sampling'Applicable for small, homogenous and readily available populationsUsed in clinical trialsMethods of Simple random sampling* Lottery method* Random number tables* Computer softwareSystematic Random Sampling:Based on sampling fraction: Every Kth unit is chosen in the population list, where K is chosen by sampling intervalQSampling Interval (K) = Total no. of units in population/ Total no. of units in sampleApplicable for large, non-homogenous populations where complete list of individuals is availableFor example, if there is a population of 1000 from which sample of 20 is to be chosen, then K = 1000/20 - 50; thus every 50th unit will be included in the sample (i.e. 1st, 51st, 101st, so on...) First unit among first 50 is chosen by simple random sampling.Stratified Random Sampling:Non-homogenous population is converted to homogenous groups/classes (strata); sample is drawn from each strata at random, in proportion to its sizeQApplicable for large non-homogenous populationQGives more representative sample than simple random samplingQNone of the categories is under or over-representedQFor example. In a population of 1000, sample of 100 is to be drawn for Hemoglobin estimation; first convert non-homogenous population is converted to homogenous strata (i.e. 700 males and 300 females), then draw 70 males and 30 females randomly respectively.Multistage Random Sampling:Is done in successive stages: each successive sampling unit is nested in the previous sampling unitQAdvantage: Introduces flexibility in samplingQFor example, in large country surveys, states are chosen, then districts, then villages, then every 10th person in village as final sampling unitMultiphase Random Sampling:Is done in successive phases; part of information is obtained from whole sample and part from the sub-sampleQFor example, in a TB survey, Mantoux test done in first phase, then X-ray done in all Mantoux positives, then sputum examined in all those with positive X-ray findings.Cluster Random Sampling:Applicable when units of population are natural groups or clustersUse in India: Evaluation of immunization coverageQWHO technique used: 30 x 7 technique (total =210 children)QWHO technique used in CRS: 30 x 7 technique (total = 210 children)30 clusters, each containing 7 children, who are 12-23 months age and are completely immunized for primary immunization (till Measles vaccine)QClusters are heterogeneous within themselves but homogenous with respect to each other Sampling interval is also calculated in CRSAccuracyQ: Low error rate of only +- 5%Limitation: Clusters cannot be compared with each otherQTypes of Non-Random SamplingConvenience Sampling:Patients are selected, in part or in whole, at the convenience of the researcher; no/limited attempt to ensure that sample is an accurate representation of populationQFor example, standing at a shopping mall and selecting shoppers as they walk by to fill out a survey.Quota Sampling:Population is first segmented into mutually exclusive sub-groups (quotas), just as in stratified sampling; then judgment is used to select the units from each group non-randomlyIs a type of convenience sampling.Snow-ball Sampling:A technique for developing a research sample where existing study subjects recruit future subjects from among their acquaintances; thus the sample group appears to grow like a rolling snowballIs often used in hidden populations which are difficult for researchers to access, e.g. drug users or commercial sex workersQClinical Trial Sampling
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