What is sampling?What is sampling?

Sampling is the process of selecting a Sampling is the process of selecting a few (sample) from a bigger group few (sample) from a bigger group

(sampling population) so that (sampling population) so that estimation or prediction is made with estimation or prediction is made with

regard to the prevalence of a particular regard to the prevalence of a particular unknown piece of information, situation unknown piece of information, situation or outcome concerning the big group or outcome concerning the big group

SamplingSampling

Basically a sample is a subgroup of the Basically a sample is a subgroup of the population in which one is interested. The population in which one is interested. The population or study population is usually population or study population is usually denoted by the letter (N) and sample size denoted by the letter (N) and sample size is (n). is (n). Researchers work with samples rather Researchers work with samples rather than populations because it is more than populations because it is more economical and practical. It is very time economical and practical. It is very time consuming and requires a lot of resources. consuming and requires a lot of resources.

Why is sampling important?Why is sampling important?

Two main reasonsTwo main reasonsFirstly subject of our enquiry is usually Firstly subject of our enquiry is usually people who are extremely problematic people who are extremely problematic unlike inanimate subjects. People are unlike inanimate subjects. People are complex, unpredictable, they cluster in complex, unpredictable, they cluster in groups, determined by social groups or groups, determined by social groups or specific interest and they are non specific interest and they are non responders, often refuse to provide us responders, often refuse to provide us information we seekinformation we seek

Why sampling is important?Why sampling is important?

Secondly the population we seek to study are Secondly the population we seek to study are frequently huge and larger the population being frequently huge and larger the population being studied, the greater the risk that a sample studied, the greater the risk that a sample drawn from that population may be drawn from that population may be unrepresentative. unrepresentative.

Because of size, cost time or lack of Because of size, cost time or lack of accesibility often makes it impossible for accesibility often makes it impossible for researchers to collect data directly from the researchers to collect data directly from the entire group of interest.entire group of interest.

SOURCE OF SAMPLESOURCE OF SAMPLE

If a research is to be applicable and If a research is to be applicable and relevant to other population, the study relevant to other population, the study sample must be representative of the sample must be representative of the group from which it is drawn, which in group from which it is drawn, which in turn should be typical of the wider turn should be typical of the wider population to whom the researcher population to whom the researcher might applymight apply..

SAMPLE SIZESAMPLE SIZESample size matters in order to have sufficient Sample size matters in order to have sufficient power to detect a meaningful result at a certain power to detect a meaningful result at a certain level of statistical significance.level of statistical significance.

Generalisability is possible depending upon the Generalisability is possible depending upon the size of the sample, how representative it is of size of the sample, how representative it is of the wider population. The larger the sample, the the wider population. The larger the sample, the more confidence we might have in generalising more confidence we might have in generalising the findingsthe findings

As a rule, the larger the sample, the more As a rule, the larger the sample, the more accurate will be the findingsaccurate will be the findings

Quantitative samplingQuantitative sampling

Quantitative and qualitative researchers Quantitative and qualitative researchers have different approaches to sampling. have different approaches to sampling.

Quantitative select samples that allow Quantitative select samples that allow researchers to generalize their results to a researchers to generalize their results to a target population and to do this, the target population and to do this, the sample must be representative. sample must be representative.

Sample must be largeSample must be large

Sample must be randomly selected.Sample must be randomly selected.

Qualitative samplingQualitative sampling

Although not exclusively, Qualitative research Although not exclusively, Qualitative research typically employs non probability sampling.typically employs non probability sampling.This means that it is not usually intended that This means that it is not usually intended that the findings of a particular study will be the findings of a particular study will be generalisable.generalisable.It will apply only to the specific population under It will apply only to the specific population under investigationinvestigationSample size is not determined by the need to Sample size is not determined by the need to ensure generalisability but a desire to fully ensure generalisability but a desire to fully investigate the chosen topic and provide rich investigate the chosen topic and provide rich datadata

Qualitative samplingQualitative sampling

In qualitative research, since the aim is to In qualitative research, since the aim is to either to explore or describe phenomena, either to explore or describe phenomena, quantification has little significance. quantification has little significance.

Researchers can find if the results are Researchers can find if the results are applicable outside the research situation applicable outside the research situation and would the findings have meaning to and would the findings have meaning to others in a similar situation.others in a similar situation.

Sample is small but generate a lot of dataSample is small but generate a lot of data

AIMS IN SELECTING A SAMPLEAIMS IN SELECTING A SAMPLE

1. To achieve maximum precision 1. To achieve maximum precision in your estimates within a given in your estimates within a given sample sizesample size

2.To avoid bias in the selection of 2.To avoid bias in the selection of your sample your sample

TYPES OF SAMPLINGTYPES OF SAMPLING

Random/probability samplingsRandom/probability samplings

Non random/ non-probability Non random/ non-probability samplingsampling

‘‘Mixed’ samplingMixed’ sampling

SAMPLING STRATEGIESSAMPLING STRATEGIES

Probability (Random) samplingProbability (Random) sampling1.1. Simple random Simple random (selection at random)(selection at random)

2.2. Systematic (selecting every nth case)Systematic (selecting every nth case)3.3. Stratified (sampling within groups of Stratified (sampling within groups of

population)population)4.4. Cluster (surveying whole clusters of Cluster (surveying whole clusters of

population sampled at random)population sampled at random)5.5. Stage (sampling clusters sampled at Stage (sampling clusters sampled at

random)random)

Selection of a sampleSelection of a sample

Simple random sampleSimple random sample– Pulling names out of a hatPulling names out of a hat

Stratified random sampleStratified random sample– Separating the units into strata (layers), e.g. age, Separating the units into strata (layers), e.g. age,

disease, gender. Including each of these strata in the disease, gender. Including each of these strata in the sample selected.sample selected.

Systematic random sampleSystematic random sample– Uses systematic intervals e.g. every 9Uses systematic intervals e.g. every 9 thth person, every person, every

33rdrd house. house.

Cluster random sampleCluster random sample– Selecting a cluster, e.g. 20 hospitals, and then choosing Selecting a cluster, e.g. 20 hospitals, and then choosing

8 to study.8 to study.

NON PROBABILITY SAMPLINGNON PROBABILITY SAMPLING

Convenience (those most convenient) also Convenience (those most convenient) also known as accidental samplingknown as accidental samplingVoluntary (Sample is self selected)Voluntary (Sample is self selected)Quota Sampling (Convenience sampling within Quota Sampling (Convenience sampling within groups of population)groups of population)Purposive sampling (Handpicked supposedly Purposive sampling (Handpicked supposedly typical or interesting casestypical or interesting casesDimensional (Multidimensional quota sampling)Dimensional (Multidimensional quota sampling)Snowball (Building up a sample through Snowball (Building up a sample through informantsinformants

Key terms of samplingKey terms of sampling

Probability sampling methodsProbability sampling methodsSimple Random SamplingSimple Random Sampling

Stratified Random samplingStratified Random sampling

Systematic Random samplingSystematic Random sampling

Cluster random samplingCluster random sampling

Random route samplingRandom route sampling

Simple Random SamplingSimple Random Sampling

Each member of a population has an Each member of a population has an equal chance of being drawn.equal chance of being drawn.

Sampling is truly random and is based on Sampling is truly random and is based on a comprehensive sampling framea comprehensive sampling frame

Quota samplingQuota sampling

Looks like stratified sampling on the face Looks like stratified sampling on the face of it.of it.It is non probability samplingIt is non probability samplingSubjects are selected in a such a manner Subjects are selected in a such a manner that each stratum of the population is that each stratum of the population is proportionately representedproportionately representedResearcher ensure that a sample of male Researcher ensure that a sample of male and female from certain ethnic groups , and female from certain ethnic groups , age, occupations are selectedage, occupations are selected

Snowball samplingSnowball sampling

Also known as nominated samplingAlso known as nominated sampling

It is non probability sampling in which It is non probability sampling in which subjects are asked to provide referrals to subjects are asked to provide referrals to other study subjectsother study subjects

Respondents are believed to have Respondents are believed to have pertinent information and are asked to pertinent information and are asked to nominate others who might be able nominate others who might be able provide further informationprovide further information

Convenience samplingConvenience sampling

Also known as accidental sampling and it Also known as accidental sampling and it is a non probability samplingis a non probability sampling

Subjects are selected for a particular study Subjects are selected for a particular study because they simply availablebecause they simply available

They are in the right place and at the right They are in the right place and at the right time and it is convenient for the time and it is convenient for the researcher’s purposeresearcher’s purpose

Purposive samplingPurposive sampling

Also termed judgemental sampling . It is a Also termed judgemental sampling . It is a type of non probability sampling in which type of non probability sampling in which subject are selected because they are subject are selected because they are identified as knowledgeable with regard to identified as knowledgeable with regard to the topic under investigationthe topic under investigation

The subjects selected are a typical group The subjects selected are a typical group from a certain area or from a certain area or

Theoretical samplingTheoretical sampling

This a non probability sampling most often This a non probability sampling most often associated with qualitative research associated with qualitative research primarily with grounded theoryprimarily with grounded theory