Nonprobability Sampling DesignsNonprobability Sampling DesignsNonprobability Sampling DesignsNonprobability Sampling Designs

Major IssuesMajor IssuesMajor IssuesMajor Issues

• Likely to misrepresent the population• May be difficult or impossible to detect

this misrepresentation

Types of Nonprobability SamplesTypes of Nonprobability SamplesTypes of Nonprobability SamplesTypes of Nonprobability Samples

Accidental, haphazard, convenienceAccidental, haphazard, convenience Modal instanceModal instance PurposivePurposive ExpertExpert QuotaQuota SnowballSnowball Heterogeneity samplingHeterogeneity sampling

Accidental, Haphazard or Accidental, Haphazard or Convenience SamplingConvenience Sampling

Accidental, Haphazard or Accidental, Haphazard or Convenience SamplingConvenience Sampling

““Man on the street”Man on the street” College psychology majorsCollege psychology majors Available or accessible clientsAvailable or accessible clients Volunteer samplesVolunteer samples Problem: Problem: NoNo evidence for evidence for

representativenessrepresentativeness

Modal Instance SamplingModal Instance SamplingModal Instance SamplingModal Instance Sampling

• Sample for the typical case• Will it play in Peoria?• Typical voter?• Problem: May not represent the modal

group proportionately

Purposive SamplingPurposive SamplingPurposive SamplingPurposive Sampling

Might sample several pre-defined Might sample several pre-defined groups (e.g., the shopping mall survey groups (e.g., the shopping mall survey that attempts to identify relevant market that attempts to identify relevant market segments)segments)

Deliberately sampling an Deliberately sampling an extremeextreme group group Problem: ProportionalityProblem: Proportionality Problem: Need theory to correctly Problem: Need theory to correctly

sample an extreme groupsample an extreme group

Expert SamplingExpert SamplingExpert SamplingExpert Sampling

Have a panel of experts make a Have a panel of experts make a judgment about the representativeness judgment about the representativeness of your sample.of your sample.

Advantage: At least you can say that Advantage: At least you can say that expert judgment supports the sampling.expert judgment supports the sampling.

Problem: The “experts” may be wrong.Problem: The “experts” may be wrong.

Quota SamplingQuota SamplingQuota SamplingQuota Sampling

Select people nonrandomly according to Select people nonrandomly according to some quotassome quotas

Proportional quota samplingProportional quota sampling Nonproportional quota samplingNonproportional quota sampling

Proportional Quota SamplingProportional Quota SamplingProportional Quota SamplingProportional Quota Sampling

• Objective: Represent major characteristics of population by sampling a proportional amount of each. For example, if you know the population has 40% women and 60% men, you want your sample to meet that quota.

• Problem: How do you pick the characteristics? How do you know their proportion in population?

Nonproportional Quota SamplingNonproportional Quota SamplingNonproportional Quota SamplingNonproportional Quota Sampling

• Making sure you have enough units from each target group of interest (even if not proportional).

• As with stratified random sampling, you might do this to assure that you have good representation of smaller population groups.

Snowball SamplingSnowball SamplingSnowball SamplingSnowball Sampling

One person recommends another, who One person recommends another, who recommends another, who recommends another, who recommends another, etc.recommends another, etc.

Good way to identify hard-to-reach Good way to identify hard-to-reach populations, for example, homeless populations, for example, homeless personspersons

Heterogeneity SamplingHeterogeneity SamplingHeterogeneity SamplingHeterogeneity Sampling

• Make sure you include all sectors -- at least several of everything -- don't worry about proportions (like in quota sampling).

• Use when one or more people are a good proxy for the group, for instance, when brainstorming issues across stakeholder groups.