E12 values for resistors

Sampling techniques: Advantages and disadvantagesTechniqueDescriptionsAdvantagesDisadvantagesSimple randomRandom sample from whole populationHighly representative if all subjects participate; the idealNot possible without complete list of population members; potentially uneconomical to achieve; can be disruptive to isolate members from a group; time-scale may be too long, data/sample could changeStratified randomRandom sample from identifiable groups (strata), subgroups, etc.Can ensure that specific groups are represented, even proportionally, in the sample(s) (e.g., by gender), by selecting individuals from strata listMore complex, requires greater effort than simple random; strata must be carefully definedClusterRandom samples of successive clusters of subjects (e.g., by institution) until small groups are chosen as unitsPossible to select randomly when no single list of population members exists, but local lists do; data collected on groups may avoid introduction of confounding by isolating membersClusters in a level must be equivalent and some natural ones are not for essential characteristics (e.g., geographic: numbers equal, but unemployment rates differ)StageCombination of cluster (randomly selecting clusters) and random or stratified random sampling of individualsCan make up probability sample by random at stages and within groups; possible to select random sample when population lists are very localizedComplex, combines limitations of cluster and stratified random samplingPurposiveHand-pick subjects on the basis of specific characteristicsEnsures balance of group sizes when multiple groups are to be selectedSamples are not easily defensible as being representative of populations due to potential subjectivity of researcherQuotaSelect individuals as they come to fill a quota by characteristics proportional to populationsEnsures selection of adequate numbers of subjects with appropriate characteristicsNot possible to prove that the sample is representative of designated populationSnowballSubjects with desired traits or characteristics give names of further appropriate subjectsPossible to include members of groups where no lists or identifiable clusters even exist (e.g., drug abusers, criminals)No way of knowing whether the sample is representative of the populationVolunteer, accidental, convenienceEither asking for volunteers, or the consequence of not all those selected finally participating, or a set of subjects who just happen to be availableInexpensive way of ensuring sufficient numbers of a studyCan be highly unrepresentativeSource: Black, T. R. (1999). Doing quantitative research in the social sciences: An integrated approach to research design, measurement, and statistics. Thousand Oaks, CA: SAGE Publications, Inc. (p. 118)