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Sampling Fundamentals 1Sampling Fundamentals 1
Sampling Fundamentals
• Population
• Sample
• Census
• Parameter
• Statistic
The One and Only Goal in Sampling!!
Select a sample that is as representative as possible.
So that an accurate inference about the population can be made – goal of marketing research
Sampling Fundamentals
• When Is Census Appropriate?
• When Is Sample Appropriate?
Error in Sampling• Total Error
• Sampling Error
• Non-sampling Error (dealt with in chapter 4)
Sampling Process: Identify Population
• Question: For a toy store in RH
• Question: For a small bookstore in RH specializing in romance novels
Sampling Process: Determine sampling frame• List and contact information of population
members used to obtain the sample from• Example – to address a population of all
advertising agencies in the US, the sampling frame would be the Standard Directory of Advertising Agencies
• Availability of lists is limited, lists may be obsolete and incomplete
Problems with sampling frames
• Subset problem– The sampling frame is smaller than the population
• Superset problem– Sampling frame is larger than the population
• Intersection problem– A combination of the subset and superset problem
Problems with sampling frames
Sampling Process: Sampling ProcedureProbability Sampling
Nonprobability Sampling
Sampling Procedure
Sampling Procedures
Non-Probability Sampling
Probability Sampling
-Simple Random Sampling-Systematic Sampling-Stratified Sampling-Cluster Sampling
-Convenience Sampling-Judgmental Sampling-Snowball Sampling-Quota Sampling
Here’s the difference!
Probabil i ty Sampling: Each subject has the same non-zero probabil i ty of getting into the sample!
Probability Sampling TechniquesSimple Random Sampling
• Each population member has equal, non-zero probability of being selected
• Equivalent to choosing with replacement
Probability Sampling Techniques
• Accuracy – cost trade off
• Sampling Efficiency = Accuracy/Cost– Sampling efficiency can be increased by
either reducing the cost, increasing the accuracy or doing both
– This has led to modifying simple random sampling procedures
Probability Sampling TechniquesStratified Sampling• The chosen sample is forced to contain units from each of
the segments or strata of the population• Sometimes groups (strata) are naturally present in the
population• Between-group differences on the variable of interest are
high and within-group differences are low• Then it makes better sense to do simple random sampling
within each group and vary within-group sample size according to– Variation on variable of interest– Cost of generating the sample– Size of group in population
• Increases accuracy at a faster rate than cost
Stratified Sampling – what strata are naturally present
Consumer type Group size 10 Percent directly proportional stratified sample size
Brand-loyal 400 40
Variety-seeking 200 20
Total 600 60
Directly Proportionate Stratified Sampling
• 600 consumers in the population:
• 200 are heavy drinkers
• 400 are light drinkers.
• If heavy drinkers opinions are valued more and a sample
size of 60 is desired, a 10 percent inversely proportional
stratified sampling is employed. Selection probabilities are computed as follows:
Denominator
Heavy Drinkers proportion and sample size
Light drinkers proportion and sample size
600/200 + 600/400 = 3 + 1.5 = 4.5
3/ 4.5 = 0.667; 0.667 * 60 = 40
1.5 / 4.5 = 0.333; 0.333 * 60 = 20
Inversely Proportional Stratified Sampling
Probability Sampling Techniques
Cluster Sampling• Involves dividing population into clusters
• Random sample of clusters is selected and all members of a cluster are interviewed
• Advantages– Decreases cost at a faster rate than accuracy
– Effective when sub-groups representative of the population can be identified
Cluster Sampling
• Math knowledge of all middle school children in the US
• Attitudes to cell phones amongst all college students in the US
• Knowledge of credit amongst all freshman college students in the US
A Comparison of Stratified and Cluster Sampling
Stratif ied sampling
Homogeneity within group
Heterogeneity between groups
All groups are included
Random sampling in each group
Sampling efficiency improved by increasing accuracy at a faster rate than cost
Cluster sampling
Homogeneity between groups
Heterogeneity within groups
Random selection of groups
Census within the group
Sampling efficiency improved by decreasing cost at a faster rate than accuracy.
Probability Sampling Techniques• Systematic Sampling
– Systematically spreads the sample through the entire list of population members
– E.g. every tenth person in a phone book– Bias can be introduced when the members in the list are
ordered according to some logic. E.g. listing women members first in a list at a dance club.
– If the list is randomly ordered then systematic sampling results closely approximate simple random sampling
– If the list is cyclically ordered then systematic sampling efficiency is lower than that of simple random sampling
Non-Probability Sampling• Benefits
– Driven by convenience
– Costs may be less
• Common Uses– Exploratory research
– Pre-testing questionnaires
– Surveying homogeneous populations
– Operational ease required
Non-Probability Sampling Techniques
• Judgmental
• Snowball
• Convenience
• Quota