SamplingSampling

An Introduction to Scientific An Introduction to Scientific Research Methods in Research Methods in

GeographyGeography

GEOG 4020GEOG 4020

SamplingSampling Without the ability to access all entities Without the ability to access all entities

available to research, scientists are available to research, scientists are necessarily limited to small numbers of necessarily limited to small numbers of cases, places, times, variables, and cases, places, times, variables, and measures. measures.

Sampling – the way of selecting a subset Sampling – the way of selecting a subset from the populationfrom the population

Virtually anything can be understood as a Virtually anything can be understood as a sample from a larger set of possible choices.sample from a larger set of possible choices.

Introduces complexity and uncertainty to Introduces complexity and uncertainty to the research processthe research process

Sampling FrameSampling Frame

Sampling frame – the subset of the Sampling frame – the subset of the population from which cases are drawn population from which cases are drawn to become part of the sampleto become part of the sample

Example: when sampling from a list of Example: when sampling from a list of registered voters, a researcher solicits registered voters, a researcher solicits participation through phone calls. participation through phone calls. The sampling frame is those on the list The sampling frame is those on the list

whose phone number is correct, are whose phone number is correct, are available to receive the phone call, and are available to receive the phone call, and are willing to participate. willing to participate.

Sampling DesignSampling Design Sampling Design – the procedure used to Sampling Design – the procedure used to

identify cases from the sampling frame to identify cases from the sampling frame to go into the samplego into the sample

Nonprobability sampling Nonprobability sampling Convenience samplingConvenience sampling Snowball samplingSnowball sampling

Probability samplingProbability sampling Simple random samplingSimple random sampling Systematic random samplingSystematic random sampling Stratified random samplingStratified random sampling Cluster samplingCluster sampling Multistage area samplingMultistage area sampling

ImplicationsImplications

Representativeness – the degree to which Representativeness – the degree to which the smaller set (sampling frame or sample) the smaller set (sampling frame or sample) resembles the larger set (population or resembles the larger set (population or sampling frame, respectively)sampling frame, respectively)

Generalizability – what larger set can Generalizability – what larger set can conclusions be validly drawn about from conclusions be validly drawn about from the evidence of the smaller set?the evidence of the smaller set?

Shortcomings of sampling can be more or Shortcomings of sampling can be more or less important in different research and less important in different research and are typically not fatal flaws. are typically not fatal flaws.

Nonparticipation and Nonparticipation and Volunteer BiasesVolunteer Biases

Nonparticipation bias – the sample becomes less Nonparticipation bias – the sample becomes less representative of the sampling frame when representative of the sampling frame when cases given the opportunity to participate cases given the opportunity to participate choose not to do so. The bias becomes greater choose not to do so. The bias becomes greater when there is a difference between participants when there is a difference between participants and nonparticipants.and nonparticipants. Record number and characteristics of nonparticipantsRecord number and characteristics of nonparticipants Maximize participationMaximize participation

RemindersReminders IncentivesIncentives Shorten, clarify a study or make a study more interesting or Shorten, clarify a study or make a study more interesting or

relevantrelevant

Volunteer bias – cases select themselvesVolunteer bias – cases select themselves

Spatial Sampling From Spatial Sampling From Continuous FieldsContinuous Fields

Spatial sampling of points from fieldsSpatial sampling of points from fields

ContinuedContinued

Independent vs. nonindependent Independent vs. nonindependent (dependent) spatial sampling – (dependent) spatial sampling – sampling on the basis of trends in sampling on the basis of trends in the datathe data

Spatial interpolation – The method Spatial interpolation – The method to make inferences from samples to make inferences from samples back to the continuous fieldback to the continuous field

Sample SizeSample Size The compromise: larger samples are more The compromise: larger samples are more

likely to be representative of the sample likely to be representative of the sample frame, however they are more likely to cost frame, however they are more likely to cost more money, time, and effort to obtain. more money, time, and effort to obtain.

Some studies require one case to make a Some studies require one case to make a conclusion (is it possible for an artificial reef conclusion (is it possible for an artificial reef to sustain populations of fish species?)to sustain populations of fish species?)

Sometimes thousands are required (political Sometimes thousands are required (political polling)polling)

Studies testing causal relationships Studies testing causal relationships (regression) require 20-30 or more. (regression) require 20-30 or more. More variables require more samples for the More variables require more samples for the

same level of confidencesame level of confidence