Presentation on theme: "University of Central Florida"— Presentation transcript:
1 University of Central Florida Chapter 12Basic Sampling IssuesCarl McDaniel, Jr.Roger GatesSlides Prepared byBruce R. BarringerUniversity of Central Florida
2 Learning Objectives Slide 1 of 2 To understand the concept of sampling.To learn the steps in developing a sampling plan.To understand the differences between probability samples and nonprobability samples.To understand the concepts of sampling error and nonsampling error.
3 Learning Objectives Slide 2 of 2 To review the types of probability sampling methods.To gain insight into nonprobability sampling methods.
4 Definitions of Important Terms Population or UniverseThe total group of people from whom information is needed.CensusData obtained from or about every member of the population of interest.SampleA subset of the population of interest.
6 Steps in Developing a Sampling Plan Slide 1 of 8 Step 1: Defining the Population of InterestBases for defining the population of interest include:GeographyDemographicsUseAwareness
7 Steps in Developing a Sampling Plan Slide 2 of 8 Step 2: Choosing a Data Collection MethodThe selection of a data collection method has implications for the sampling process.Step 3: Choosing a Sampling FrameSampling frameList of population elements from which to select units to be sampled.
8 Steps in Developing a Sampling Plan Slide 3 of 8 Step 4: Selecting the Sampling MethodProbability samplesSamples in which every element of the population has a known, nonzero probability of selection.Nonprobability samplesInclude the selection of specific elements from the population in a nonrandom manner.
9 Steps in Developing a Sampling Plan Slide 4 of 8 Step 4: Selecting the Sampling Method (continued)Sampling error: The difference between the sample value and the true value of the population mean.
10 Steps in Developing a Sampling Plan Slide 5 of 8 Advantages of probability samplesDisadvantages of probability samples- The researcher can be sure ofobtaining information from arepresentative cross section of thepopulation of interest.- Sampling error can be computed.- The survey results are projectableto the total population.- They are more expensive thannonprobability samples of thesame size in most cases. Therules for selection increaseinterviewing costs and professionaltime must be spent in developingthe sample design.- Probability samples take more timeto design and execute than non-probability samples.
11 Steps in Developing a Sampling Plan Slide 6 of 8 Advantages of nonprobability samplesDisadvantages of nonprobability samples- Nonprobability samples cost lessthan probability samples. Thischaracteristic of nonprobabilitysamples may have considerableappeal in those situations whereaccuracy is not of criticalimportance.- Nonprobability samples ordinarilycan be conducted more quicklythan probability samples.- Sampling error cannot be computed.- The researcher does not know thedegree to which the sample isrepresentative of the populationfrom which it was drawn, but can draw inferences.- The results of nonprobabilitysamples cannot and should not beprojected to the total population; this is not true in all cases.
12 Steps in Developing a Sampling Plan Slide 7 of 8 Step 5: Determine Sample SizeOnce the sampling method has been chosen, the next step is to determine the appropriate sample size.Step 6: Developing Operational Procedures for Selecting Sample ElementsInvolves determining whether a probability or nonprobability sample is being used.
13 Steps in Developing a Sampling Plan Slide 8 of 8 Step 7: Execution of the Sampling PlanThe final step in the sampling process involves execution of the operational sampling plan discussed in the previous steps.It is important that this step include adequate checking to make sure that specified procedures are adhered to.
14 Classification of Sampling Methods ProbabilitySamplesNon-probabilitySystematicStratifiedConvenienceSnowballClusterSimpleRandomJudgmentQuota
15 Probability Sampling Methods Slide 1 of 4 Simple Random SamplingIs considered to be the purest form of probability sampling. A probability sample is a sample in which every element of the population has a known and equal probability of being selected into the sample.Sample SizeProbability of Selection =Population Size
16 Probability Sampling Methods Slide 2 of 4 Systematic SamplingProbability sampling in which the entire population is numbered, and elements are drawn using a skip interval.Population SizeSkip Interval =Sample Size
17 Probability Sampling Methods Slide 3 of 4 Stratified SamplesStratified samples are probability samples that are distinguished by the following procedural steps:First, the original or parent population is divided into two or more mutually exclusive and exhaustive subsets (e.g., male and female).Second, simple random samples of elements from the two or more subsets are chosen independently from each other, either proportionately or disproportionately.
18 Probability Sampling Methods Slide 4 of 4 Cluster SamplesIn the case of cluster samples, the sampling units are selected in groups. There are two basic steps in cluster sampling:First, the population of interest is divided into mutually exclusive and exhaustive subsets.Second, a random sample of the subsets is selected.
19 Nonprobability Sampling Methods Slide 1 of 3 Convenience SamplesNonprobability samples used primarily because they are easy to collect.Judgment SamplesNonprobability samples in which the selection criteria are based on personal judgment that the element is representative of the population under study.
20 Nonprobability Sampling Methods Slide 2 of 3 Quota SamplesNonprobability samples in which population subgroups are classified on the basis of researcher judgment.Snowball SamplesNonprobability samples in which selection of additional respondents is based on referrals from the initial respondents.
21 Nonprobability Sampling Methods Slide 3 of 3 Convenience SamplesNonprobability samples used primarily because they are easy to collect.Judgment SamplesNonprobability samples in which the selection criteria are based on personal judgment that the element is representative of the population under study.
22 Summary of Key Points Slide 1 of 2 The population, or universe, is the total group of people in whose opinions one is interested.A census involves collecting desired information from all the members of the population of interest.A sample is simply a subset of a population.
23 Summary of Key Points Slide 2 of 2 Probability sampling methods are selected in such a way that every element of the population has a known, nonzero probability of selection.Nonprobability sampling methods include all methods that select specific elements from the population in a nonrandom manner.