Discussion Sampling Discussion Sampling MethodsMethods

GIS Group MeetingGIS Group MeetingSan Lwin Htwe

GIS Specialist (UNODC)

What is Sampling?What is Sampling?

the process of selecting a the process of selecting a small number of elements small number of elements from a larger defined target from a larger defined target group of elementsgroup of elements

the information gathered the information gathered from the small group will from the small group will allow judgments to be made allow judgments to be made about the larger groupsabout the larger groups

Why sampling?Why sampling?

Inability to analyze large Inability to analyze large quantities of data potentially quantities of data potentially generated by a population generated by a population

Practical considerations such as Practical considerations such as cost and timecost and time

Sampling can produce sound Sampling can produce sound results if proper rules are results if proper rules are followed for the drawfollowed for the draw

Basic concepts of Basic concepts of SamplingSampling

PopulationPopulation SampleSample Sampling unitSampling unit Sampling errorSampling error Sampling frameSampling frame Sampling sizeSampling size

Basic concepts of Basic concepts of SamplingSampling

Population:Population: baseline -baseline -the entire the entire group under study as group under study as defined by objectivesdefined by objectives

Sample:Sample: a subset of a subset of the population that the population that should represent the should represent the entire groupentire group

Basic concepts of Basic concepts of SamplingSampling Sample unit:Sample unit: the basic level of investigation the basic level of investigation

Sampling error:Sampling error:

same sampling methods, same population, the same sampling methods, same population, the study with a larger sample size will have less study with a larger sample size will have less sampling process error compared to the study sampling process error compared to the study with smaller sample sizewith smaller sample size

Basic concepts of Basic concepts of SamplingSampling Sampling frame:Sampling frame:

a master list of the a master list of the population (total or population (total or partial) from which partial) from which the sample will be the sample will be drawndrawn

Sampling size:Sampling size: number of samples number of samples to be drawnto be drawn

Types of SamplingTypes of Sampling Probability Probability

samplingsampling

in which members in which members of the population of the population have a known have a known chance chance (probability) of (probability) of being selected being selected

Non-probability Non-probability samplingsampling

in which the in which the chances chances (probability) of (probability) of selecting selecting members from members from the population are the population are unknownunknown

Sampling methodsSampling methods

ProbabilityProbability Simple random Simple random

samplingsampling Stratified random Stratified random

samplingsampling Systematic Systematic

samplingsampling Cluster samplingCluster sampling

Non-probabilityNon-probability Convenience Convenience

samplingsampling Judgment Judgment

samplingsampling Quota samplingQuota sampling Snowball Snowball

samplingsampling

Probability Sampling Probability Sampling MethodsMethods

Simple random Simple random methodmethodevery unit has an equal every unit has an equal non-zero chance of being non-zero chance of being selectedselected

Advantages: Advantages: Known and equal Known and equal

chance of selectionchance of selection Easy method when Easy method when

there is an electronic there is an electronic databasedatabase

Disadvantages: Disadvantages: Complete accounting Complete accounting

of population neededof population neededThis method is the purest form of probability sampling

Probability Sampling Probability Sampling MethodsMethods

Stratified random Stratified random methodmethodthe population is the population is separated into separated into homogeneous strata and homogeneous strata and a sample is taken from a sample is taken from eacheach

Advantages:Advantages:

More accurate overall More accurate overall sample of skewed sample of skewed populationpopulation

Disadvantages: Disadvantages: More complex More complex

sampling plan sampling plan requiring different requiring different sample sizes for each sample sizes for each stratumstratumOften used when one or more of the stratums in the

population have a low incidence relative to the other stratums.

Probability Sampling Probability Sampling MethodsMethods

Systematic Systematic methodmethodthe defined target the defined target population is ordered and population is ordered and the sample is selected the sample is selected according to position using according to position using a skip intervala skip interval

Advantages: Advantages: Known and equal Known and equal

chance of selected chance of selected intervalinterval

Less expensive…faster Less expensive…faster than Radom methodsthan Radom methods

Disadvantages: Disadvantages: Loss in sampling Loss in sampling

precisionprecisionSystematic sampling is frequently used to select a specified number of records from a computer file.

Probability Sampling Probability Sampling MethodsMethodsCluster methodCluster methodthe population is divided the population is divided into groups (clusters), any into groups (clusters), any of which can be of which can be considered a considered a representative samplerepresentative sample

Advantages:Advantages:

Economic efficiency … Economic efficiency … faster and less faster and less expensiveexpensive

Does not require a list Does not require a list of all members of the of all members of the populationpopulation

Disadvantages:Disadvantages:

Cluster specification Cluster specification error…error…the more homogeneous the the more homogeneous the cluster chosen, the more cluster chosen, the more imprecise the sample resultsimprecise the sample results

Non-probability Sampling Non-probability Sampling MethodsMethods

Convenience Convenience sampling sampling methodmethodthe selecting on the basis the selecting on the basis of convenience of convenience the selection at familiar the selection at familiar locations and to choose locations and to choose respondents who are like respondents who are like themselvesthemselves

Judgment Judgment methodmethodselecting samples that selecting samples that require a judgment or an require a judgment or an “educated guess”“educated guess”

often used during preliminary research efforts to get a gross estimate of the results

must be confident that the chosen sample is truly representative of the entire population.

Non-probability Sampling Non-probability Sampling MethodsMethods

Quota sampling Quota sampling methodmethodsamples that set a samples that set a specific number of certain specific number of certain types of individuals types of individuals Often used to ensure Often used to ensure desired proportion of desired proportion of different respondent different respondent classesclasses

Snowball methodSnowball methodselecting samples which selecting samples which require respondents to require respondents to provide the names of provide the names of additional respondentsadditional respondents

special method used when the desired sample characteristic is rare

Online Sampling Techniques Random online intercept sampling:Random online intercept sampling:

relies on a random selection of Web site visitorsrelies on a random selection of Web site visitors Invitation online sampling:Invitation online sampling:

is when potential respondents are alerted that is when potential respondents are alerted that they may fill out a questionnaire that is hosted they may fill out a questionnaire that is hosted at a specific Web siteat a specific Web site

Online panel sampling:Online panel sampling:refers to consumer or other respondent panels refers to consumer or other respondent panels that are set up by marketing research that are set up by marketing research companies for the explicit purpose of conducting companies for the explicit purpose of conducting online surveys with representative samplesonline surveys with representative samples

Developing a Developing a Sampling PlanSampling Plan Step-1Step-1

Define the relevant Define the relevant population (baseline)population (baseline)

Step-2Step-2Identify sample Identify sample frameframe

Step-3Step-3Determine specific Determine specific sampling method, all sampling method, all necessary steps necessary steps must be specifiedmust be specified

Step-4Step-4Determine sample Determine sample size, selecting size, selecting samplessamples

Step-5Step-5Execute the samplingExecute the sampling

Step-6Step-6SSampling validation -ampling validation -compare sample compare sample profile with population profile with population profile Re-sampling if profile Re-sampling if necessarynecessary

Factors to consider in sampling design Work objectivesWork objectives Degree of accuracyDegree of accuracy ResourcesResources Time frameTime frame Knowledge on Knowledge on

populationpopulation ScopeScope Statistical analysis Statistical analysis

needsneeds

How to determine sample size?

Common approaches for determining sample size

Budget/time available Executive decision Statistical methods Historical data/guidelines

ISBN: 8173196257

ConclusionsConclusions

A well-designed sampling plan answers the following questions –What will be learned?How long will it take?How much will it cost?

Sampling is important for a survey/research Sampling is important for a survey/research projectproject

Many sampling start with a general hope that Many sampling start with a general hope that something interesting will emerge, and often end something interesting will emerge, and often end in frustrationin frustration

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