2 Population, sample and individual cases Selecting samplesPopulation, sample and individual casesSource: Saunders et al. (2009)Figure 7.1 Population, sample and individual cases
3 Sampling- a valid alternative to a census when The need to sampleSampling- a valid alternative to a census whenA survey of the entire population is impracticableBudget constraints restrict data collectionTime constraints restrict data collectionResults from data collection are needed quickly
4 Overview of sampling techniques Source: Saunders et al. (2009)Figure 7.2 Sampling techniques
5 Probability sampling The four stage process Identify sampling frame from research objectivesDecide on a suitable sample sizeSelect the appropriate technique and the sampleCheck that the sample is representative
6 Identifying a suitable sampling frame Key points to considerProblems of using existing databasesExtent of possible generalisation from the sampleValidity and reliabilityAvoidance of bias
7 Choice of sample size is influenced by Confidence needed in the dataMargin of error that can be toleratedMargin of error (also called The confidence interval ) is the plus-or-minus figure usually reported in newspaper or television opinion poll results. For example, if you use a margin of error of 4 and 47% percent of your sample picks an answer you can be "sure" that if you had asked the question of the entire relevant population between 43% (47-4) and 51% (47+4) would have picked that answer.Types of analyses to be undertakenSize of the sample population and distribution
9 The importance of response rate Key considerationsNon- respondents and analysis of refusalsObtaining a representative sampleCalculating the active response rateEstimating response rate and sample size
10 Selecting a sampling technique Five main techniques used for a probability sampleSimple randomSystematicStratified randomClusterMulti-stage
11 Simple random sampling Number each of the cases in your sampling frame with a unique number.Select cases using random numbers until, actual sample size is reached.Computer aided telephone interviewing (CATI) software
12 Systematic Random Sampling Number each of the cases in your sampling frame with a unique number.Select the first case using a random numberCalculate the sampling fractionSelect subsequent cases systematically using the sampling fraction to determine the frequency of selection.Sampling fraction = actual sample size/ total population
13 Stratified random sampling Choose the stratification variable or variablesDivide the sampling frame into the discrete strata.Number each of the cases within each stratum with a unique numberSelect your sample using either simple random or systematic random sampling
14 Cluster sampling Choose the cluster grouping for your sampling frame. Number each of the clusters with a unique number.Select sample of clusters using random sampling
18 Quota Sampling Divide the population into specific groups. Calculate quota for each group based on relevant and available dataCollect data from each quota
19 Purposive samplingExtreme case/deviant sampling: unusual or special case enable to learn the most about the RQ.Heterogeneous or maximum variation sampling: representing different subgroupsHomogeneous sampling: One subgroup.Critical case sampling:If it happen there, it will happen everywhere.
20 Snowball samplingMake contact with one or two cases in the population.Ask these cases to identify further cases.Ask these new case to identify further new cases.Stop when either no new cases are given or the sample is large enough.
21 Self select sampling Publicize your need for cases Collect data from those who respond
22 Haphazard sampling Also called purposive or availability sampling. Select case based on ease or convenience.
23 Summary: Chapter 7Choice of sampling techniques depends upon the research question(s) and their objectivesFactors affecting sample size include:- confidence needed in the findings- accuracy required- likely categories for analysis
24 Summary: Chapter 7Probability sampling requires a sampling frame and can be more time consumingWhen a sampling frame is not possible, non- probability sampling is usedMany research projects use a combination of sampling techniques
25 All choices depend on the ability to gain access to organisations Summary: Chapter 7All choices depend on the ability to gain access to organisations