Sampling Techniques

Doyinmola Ojifinni

Community Medicine Dept., University College Hospital, Ibadan

What Is Sampling?

Sampling is the process of selecting a sample from a population

Necessitated by factors such as feasibility and limited resources

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Definition of Terms

The term population is used to describe a large set or collection of items that have something in common

In medicine, population generally refers to patients or other living organisms, but the term can also be used to denote collections of inanimate objects such as autopsy reports, birth certificates etc.

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Definition of Terms

A sample is a subset of the population

Suppose a population consists of all patients in a certain ward

Measuring the heights of only a fraction of these patients for analysis is working with a sample

Sample

Population

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Definition of Terms

Sampling frame

Units of sampling

Study units or units of the investigation/enquiry

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Principles of Sampling

Avoidance of bias

Systematic deviation from the truth

Achieve high precision for the available resources

Time, funds

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Types of Sampling Techniques

Probability or random sampling the chance of selecting every unit in the population is known

Non probability or non random sampling the chance of selecting units is not known

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Examples Of Probability Samples

Simple Random Sample Systematic Sample Stratified Random Sample Cluster Sample Multi-stage Random Sample Multi-phase Random Sample

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Examples Of Non-probability Samples

Volunteers

Haphazard

Quota

Purposive

Convenience

Snowballing

Respondent driven sampling

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Simple random sampling

Every unit has an equal chance of being selected. Can be done using a table of random numbers,

balloting, computer generated list of random numbers

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Simple random sampling

Characteristics Sampling frame must be available which is a complete list of

all the units of the population

Precision of estimate is the highest of all methods

However construction of frame may be laborious and expensive

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Systematic random sampling

Every kth item is selected k is the sampling interval and is determined by dividing the number of

items in the sampling frame by the desired sample size

First subject selected using simple random methods

Should not be used when a cyclic repetition is inherent in the sampling frame

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Stratified random sampling

Population is divided into homogenous strata according to some characteristic e.g. department,

course of study etc.

Random sample selected from each stratum

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Cluster sampling

Results from a process in which the population is divided into clusters

Subset of the clusters is randomly selected

Clusters commonly based on geographic areas or districts

More common in epidemiological surveys than clinical studies

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Multistage sampling

Sampling by stages between 2 to 6 The population is divided into 1st stage units and a sample of these is selected

Each selected first stage unit is subdivided in to second stage units and a sample is these is selected.

The procedure continues until the units of enquiry are reached

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What sampling methods would you use?

Selection of 300 PLWHAs attending care at the antiretroviral treatment clinic

Selection of 340 women attending care at the Antenatal Clinic of the UCH Ibadan

Selection of 150 medical students of the University of Ibadan in clinical school

Selection of 189 street children to assess their psychoactive drug use

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Questions?

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Thank you!

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