2 What is Sampling?the process of selecting a small number of elements from a larger defined target group of elementsthe information gathered from the small group will allow judgments to be made about the larger groups
3 Why sampling?Inability to analyze large quantities of data potentially generated by a populationPractical considerations such as cost and timeSampling can produce sound results if proper rules are followed for the draw
4 Basic concepts of Sampling PopulationSampleSampling unitSampling errorSampling frameSampling size
5 Basic concepts of Sampling Population: baseline -the entire group under study as defined by objectivesSample: a subset of the population that should represent the entire group
6 Basic concepts of Sampling Sample unit: the basic level of investigationSampling error:same sampling methods, same population, the study with a larger sample size will have less sampling process error compared to the study with smaller sample size
7 Basic concepts of Sampling Sampling frame:a master list of the population (total or partial) from which the sample will be drawnSampling size: number of samples to be drawn
8 Types of Sampling Probability sampling in which members of the population have a known chance (probability) of being selectedNon-probability samplingin which the chances (probability) of selecting members from the population are unknown
9 Sampling methods Probability Simple random sampling Stratified random samplingSystematic samplingCluster samplingNon-probabilityConvenience samplingJudgment samplingQuota samplingSnowball sampling
10 Probability Sampling Methods Simple random methodevery unit has an equal non-zero chance of being selectedAdvantages:Known and equal chance of selectionEasy method when there is an electronic databaseDisadvantages:Complete accounting of population neededThis method is the purest form of probability sampling
11 Probability Sampling Methods Stratified random methodthe population is separated into homogeneous strata and a sample is taken from eachAdvantages:More accurate overall sample of skewed populationDisadvantages:More complex sampling plan requiring different sample sizes for each stratumOften used when one or more of the stratums in the population have a low incidence relative to the other stratums.
12 Probability Sampling Methods Systematic methodthe defined target population is ordered and the sample is selected according to position using a skip intervalAdvantages:Known and equal chance of selected intervalLess expensive…faster than Radom methodsDisadvantages:Loss in sampling precisionSystematic sampling is frequently used to select a specified number of records from a computer file.
13 Probability Sampling Methods Cluster methodthe population is divided into groups (clusters), any of which can be considered a representative sampleAdvantages:Economic efficiency … faster and less expensiveDoes not require a list of all members of the populationDisadvantages:Cluster specification error…the more homogeneous the cluster chosen, the more imprecise the sample results
14 Non-probability Sampling Methods Convenience sampling methodthe selecting on the basis of conveniencethe selection at familiar locations and to choose respondents who are like themselvesJudgment methodselecting samples that require a judgment or an “educated guess”often used during preliminary research efforts to get a gross estimate of the resultsmust be confident that the chosen sample is truly representative of the entire population.
15 Non-probability Sampling Methods Quota sampling methodsamples that set a specific number of certain types of individualsOften used to ensure desired proportion of different respondent classesSnowball methodselecting samples which require respondents to provide the names of additional respondentsspecial method used when the desired sample characteristic is rare
16 Online Sampling Techniques Random online intercept sampling:relies on a random selection of Web site visitorsInvitation online sampling:is when potential respondents are alerted that they may fill out a questionnaire that is hosted at a specific Web siteOnline panel sampling:refers to consumer or other respondent panels that are set up by marketing research companies for the explicit purpose of conducting online surveys with representative samples
17 Developing a Sampling Plan Step-1Define the relevant population (baseline)Step-2Identify sample frameStep-3Determine specific sampling method, all necessary steps must be specifiedStep-4Determine sample size, selecting samplesStep-5Execute the samplingStep-6Sampling validation -compare sample profile with population profile Re-sampling if necessary
18 Factors to consider in sampling design Work objectivesDegree of accuracyResourcesTime frameKnowledge on populationScopeStatistical analysis needs
20 Common approaches for determining sample size Budget/time availableExecutive decisionStatistical methodsHistorical data/guidelinesISBN:
21 ConclusionsSampling is important for a survey/research projectMany sampling start with a general hope that something interesting will emerge, and often end in frustrationA well-designed sampling plan answers the following questions –What will be learned?How long will it take?How much will it cost?