Sampling Process Sample vs. Census Sample vs. Census
The Sampling Design Process The Sampling Design Process Define the Population
Define the Population
Determine the Sampling Frame Determine the Sampling Frame Select Sampling Technique(s) Select Sampling Technique(s) Determine the Sample Size Determine the Sample Size Execute the Sampling Process Execute the Sampling Process Define the Target Population Define the Target Population
The target population is the collection of elements or objects that possess the information sought The target population is the collection of elements or objects that possess the information sought by the researcher and about which inferences are to be made.
by the researcher and about which inferences are to be made. The target population should beThe target population should be defined in terms of
defined in terms of elements, sampling units, extent, and time.elements, sampling units, extent, and time.
An
An elementelement is the object about which or from which the information is desired, e.g., theis the object about which or from which the information is desired, e.g., the respondent.
respondent.
A
A sampling unitsampling unit is an element, or a unit containing the element, that is available for selection atis an element, or a unit containing the element, that is available for selection at some stage of the sampling process.
some stage of the sampling process.
Extent
Extent refers to the geographical boundaries.refers to the geographical boundaries.
Time
Time is the time period under is the time period under consideration.consideration.
•• Determining Target PopulationDetermining Target Population
•• Well thought out research objectivesWell thought out research objectives
•• Consider all alternativesConsider all alternatives
•• Know your marketKnow your market
•• Consider the appropriate sampling unitConsider the appropriate sampling unit
•• Specify clearly what is excludedSpecify clearly what is excluded
•• Should be reproducibleShould be reproducible
•• Consider convenienceConsider convenience Determine the Sampling Frame Determine the Sampling Frame
•• If a probability sample is to be taken, a sampling frame is required.If a probability sample is to be taken, a sampling frame is required.
•• A SampliA Sampling frame is ng frame is a means of a means of representing the elements of the population.representing the elements of the population.
•• For example, telephone book, map or For example, telephone book, map or city directorycity directory..
•• A perfect sampling frame is one in which A perfect sampling frame is one in which every element of the every element of the population is representedpopulation is represented once but only once.
once but only once.
Convenience Sampling Convenience Sampling
Convenience sampling attempts to obt
Convenience sampling attempts to obtain a sample of convenient elements. ain a sample of convenient elements. Often, respondentsOften, respondents are selected because they happen to be
are selected because they happen to be in the right place at the right time.in the right place at the right time.
Conditions Favoring the Use of Conditions Favoring the Use of Type
Type of of Study Study Sample Sample CensusCensus
1. Budget
1. Budget Small Small LargeLarge
2. Time available
2. Time available Short Short LongLong 3. Population size
3. Population size Large Large SmallSmall 4. Variance in the characteristic
4. Variance in the characteristic Small Small LargeLarge 5. Cost of sampling errors
5. Cost of sampling errors Low Low HighHigh 6. Cost of nonsampling errors
6. Cost of nonsampling errors High High LowLow 7. Attention to individual cases
7. Attention to individual cases Yes Yes NoNo
–
– use of students, and members of social organizationsuse of students, and members of social organizations
–
– mall intercept interviews without qualifying the mall intercept interviews without qualifying the respondentsrespondents
–
– department stores using charge account department stores using charge account listslists
–
– “people on the street” interviews“people on the street” interviews Judgmental Sampling
Judgmental Sampling
Judgmental sampling is a form of convenience sampling in which the population elements are Judgmental sampling is a form of convenience sampling in which the population elements are selected based on
selected based on the judgment of the the judgment of the researcherresearcher..
–
– test marketstest markets
–
– purchase engineers selected in industrial marketing researchpurchase engineers selected in industrial marketing research
–
– expert witnesses used in courtexpert witnesses used in court Quota Sampling
Quota Sampling Quota sampling
Quota sampling may be viewed as may be viewed as two-stage restricted judgmental sampling.two-stage restricted judgmental sampling.
–
– The first stage consists of The first stage consists of developing control categories, or quotas, of pdeveloping control categories, or quotas, of p opulationopulation elements.
elements.
–
– In the second stage, sample elements are selected based on conIn the second stage, sample elements are selected based on con venience or venience or judgment.
judgment.
P
Pooppuullaattiioonn SSaammppllee c
coommppoossiittiioonn ccoommppoossiittiioonn Control
Control C
Chhaarraacctteerriissttiicc PPeerrcceennttaaggee PPeerrcceennttaaggee NuNummbbeer r Sex
Sex M
Maallee 4488 4488 448800
F
Feemmaallee 5522 5522 552200 _
_______ ________ _______ _ 1
10000 110000 10100000 Snowball Sampling
Snowball Sampling In
In snowball samplingsnowball sampling, an initial group of respondents is selected, usually at random., an initial group of respondents is selected, usually at random.
–
– After being interviewed, these respondents are asked After being interviewed, these respondents are asked to identify others whoto identify others who belong to the target population of interest.
belong to the target population of interest.
–
– Subsequent respondents are selected based on the referrals.Subsequent respondents are selected based on the referrals.
Simple Random Sampling Simple Random Sampling
•• Each element in the population has a known and equal Each element in the population has a known and equal probability of selection.probability of selection.
•• Each possible sample of a given size (n) has a Each possible sample of a given size (n) has a known and equal probability of being theknown and equal probability of being the sample actually selected.
sample actually selected.
•• This implies that every element is selected This implies that every element is selected independently of every other element.independently of every other element.
Systematic Sampling Systematic Sampling
•• Involves systematically spreading the sample through the list Involves systematically spreading the sample through the list of population membersof population members
•• Commonly used in telephone surveysCommonly used in telephone surveys
•• Sampling efficiency depends on ordering of the Sampling efficiency depends on ordering of the list in the sampling framelist in the sampling frame Stratified Sampling
Stratified Sampling
•• The chosen sample is forced to contain units from each of the The chosen sample is forced to contain units from each of the segments or strata of thesegments or strata of the population
population
Types of Stratified Sampling Types of Stratified Sampling
•• Proportionate Stratified SamplingProportionate Stratified Sampling
•• Number of objects/sampling units chosen from each group is proportional to number inNumber of objects/sampling units chosen from each group is proportional to number in population
population
•• Can be classified as directly Can be classified as directly proportional or indirectly proportional stratified samplingproportional or indirectly proportional stratified sampling
•• Disproportionate Stratified SamplingDisproportionate Stratified Sampling
•• Sample size in each group is not proportional to the respective group sizesSample size in each group is not proportional to the respective group sizes
•• Used when multiple groups are compared and respective group sizes are smallUsed when multiple groups are compared and respective group sizes are small Cluster Sampling
Cluster Sampling
•• Involves dividing population into subgroupsInvolves dividing population into subgroups
•• Random sample of subgroups/clusters is selected and all members of subgroups areRandom sample of subgroups/clusters is selected and all members of subgroups are interviewed
interviewed
•• Very cost effectiveVery cost effective
•• Useful when subgroups can be Useful when subgroups can be identified that are representative of entire populationidentified that are representative of entire population Determinatio
Determination of n of Sample SizeSample Size
Important qualitative factors in determining the sample size Important qualitative factors in determining the sample size
–
– the importance of the decisionthe importance of the decision
–
– the nature of the researchthe nature of the research
–
– the nature of the analysisthe nature of the analysis
–
– sample sizes used in sample sizes used in similar studiessimilar studies
–
– incidence ratesincidence rates
–
– completion ratescompletion rates
–
– resource constraintsresource constraints Execute the Sampling Process Execute the Sampling Process
The final step in the sampling process is the actual selection of the sample elements. This The final step in the sampling process is the actual selection of the sample elements. This requires a substantial amount of
requires a substantial amount of office and field work, particularly if personal interviews areoffice and field work, particularly if personal interviews are involved
involved
Issues in Multi National Sampling Issues in Multi National Sampling
•• Define the populationDefine the population
•• Specify the sampling frameSpecify the sampling frame
•• Specify the sampling unitSpecify the sampling unit
•• Selection of the sampling methodSelection of the sampling method
•• Determine the sample sizeDetermine the sample size
•• Specify the sampling planSpecify the sampling plan
•• Select the sampleSelect the sample Determining the Sample Size Determining the Sample Size
•• Unaided JudgmentUnaided Judgment
•• All you can affordAll you can afford
•• Average size of sample for similar studiesAverage size of sample for similar studies
•• Required Size per cellRequired Size per cell
•• Use of a Use of a traditional statistical modeltraditional statistical model
•• Use of a Use of a Bayesian statistical modelBayesian statistical model