pick 1 hospitals (i.e., 1 cluster), and ask the nurses from that hospital to fill out the surveyWhen to use non-probability sampling techniques?•Researcher is constrained in time or resources•For exploratory research•Target population is small or difficult to identify•Sampling error is a major concern278/29/2016
10Non-probability Sampling Techniques28Nonprobability Sampling TechniquesConvenienceSamplingJudgmentalSamplingQuotaSamplingSnowballSampling8/29/2016Convenience SamplingConvenience sampling attempts to obtain a sample of convenient elements. Often, respondents are selected because they happen to be in the right place at the right time. Examples– use of students and members of social organizations– mall intercept interviews without qualifying the respondents– Use social media to recruit respondents– “people on the street” interviews298/29/2016Judgmental SamplingJudgmental sampling is a form of convenience sampling in which the population elements are selected based on the judgment of the researcher.Examples – test markets– purchase engineers selected in industrial marketing research – expert witnesses used in court308/29/2016
11Quota SamplingQuota sampling may be viewed as two-stage restricted judgmental sampling. – The first stage consists of identifying segments in the population – In the second stage, sample elements are selected from each segment based on convenience or judgment– Analogous to stratified sampling: the main difference is in the 2ndstage, where selection is not probability-based318/29/2016Snowball SamplingIn snowball sampling, an initial group of respondents is selected, usually based on convenience– After being interviewed, these respondents are asked to identify others who belong to the target population of interest – Subsequent respondents are selected based on the referrals– Normally used to study illicit behavior (e.g., illegal immigration or drug use)328/29/2016Choosing Non-probabilityvs. Probability SamplingCONDITIONS FAVORING THE USE OFFactorsNonprobability SamplingProbability SamplingNature of researchExploratoryConclusiveRelative magnitude of sampling and non-sampling errorsNon-sampling errors are largerSampling errors are largerVariability in the populationHomogeneous(low)Heterogeneous(high)Statistical considerationsUnfavorableFavorableOperational considerationsFavorableUnfavorable338/29/2016
122.3. Deciding on the sample size348/29/2016Key trade‐offs betweenlarge vs. small sample size•Small sample size: cheaperand faster to execute; butresults are less reliable•Large sample size: moreexpensive, but more likely tobe representative of thepopulation and hence resultsare less prone to errorSome rules of thumb for sample sizes of different types of research projectType of StudyMinimum SizeTypical RangeProblem identification research(e.g., market potential)5001000 – 2500Problem solving research (e.g., pricing)200300 – 500Product tests200300 – 500Test marketing studies200300 – 500TV/radio/print advertising(per commercial or ad tested)150200 – 300Test-market audits10 stores10 – 20 storesFocus groups2 groups10 – 15 groups358/29/2016Sampling process for your survey
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