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Sampling Methods in Quantitative and Qualitative Research

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Sampling

• Sampling in Quantitative Research

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Sampling in Quantitative Research

• Population– The entire aggregation of cases that meets a specified

set of criteria• Eligibility criteria determines the attributes of the target

population

• Sampling– The process of selecting a portion of the population to

represent the entire population

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Sampling in Quantitative Research

• Accessible population– The population of people available for a study

• Target population– The entire population in which the researcher is

interested and to which he/she wants to generalize the results

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Sampling Plans

• A sample is a subset of the population

– A sample should be representative and similar to the population to be studied

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Sampling Plans

• Strata

– Subdivisions of the population based on specific characteristics

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Samples vs. the Population

• More economical• More efficient• More practical

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Problems Using Samples

• Sampling bias

– Over-representation or under-representation of some characteristic of the population

– Not representative of the population being studied

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Sampling Plans

• Types of sampling plans

– Nonprobability sample• Convenience sampling• Purposive sampling• Quota sampling

– Probability sample• Random sampling• Cluster sampling• Systematic sampling

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Sampling Plans

• Nonprobability sample

– The selection of the sample from a population using non-random procedures

• Convenience sampling• Purposive sampling• Quota sampling

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Sampling Plans

• Nonprobability sample

– Convenience sampling (accidental sampling)• Selection of the most readily available people as participants in a study• Risk of bias and errors as sample may be atypical of the population• Weakest form of sampling

– Snowball sampling (network sampling)• The selection of participants by means of referrals from earlier

participants

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Sampling Plans

• Nonprobability sample

– Quota sampling• Researcher pre-specifies characteristics of the sample to

increase its representativeness

• This is used so sample includes an appropriate number of cases from each stratum (subpopulation)

• Usually use age, gender, ethnicity, socioeconomic status, and medical diagnosis

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Sampling Plans

• Nonprobability sample

– Purposive sampling (judgmental sampling)• Researcher selects study participants on the basis of

personal judgement about which ones will be most representative or productive

• Handpick cases, very subjective

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Sampling Plans

• Nonprobability Sample Problems

– Are rarely representative of the target population– But are convenient and economical

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Sampling Plans

• Probability sample

– The selection of the sample from a population using random procedures

– Random selection – each element in the population has an equal, independent chance of being selected

– Should be representative of the population

• Random sampling• Cluster sampling• Systematic sampling

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Sampling Plans

• Probability sample

• Simple Random sampling

– Listing the population elements– Elements are assigned a number– Table of random numbers is used to draw at

random a sample

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Sampling Plans

• Probability sample

• Stratified Random sampling

– Population divided into homogenous subsets– Elements are selected at random– Increases representativeness of the final sample

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Sampling Plans

• Probability sample

– Stratified Random sampling

– Proportionate sample » a sample that results when the researcher

samples from different strata of a population in direct proportion to their representation in the population

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Sampling Plans

• Probability sample

– Stratified Random sampling

– Disproportionate sample » a sample that results when the researcher

samples differing proportions of study participants from different strata that are comparatively smaller

» Used when comparison between strata of unequal membership size are desired

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Sampling Plans

• Probability sample

– Cluster sampling (multistage sampling)

• A form of sampling in which large groupings are selected first, with successive subsampling of smaller units

• Used for large scale sampling where it is impossible to have a listing of all elements

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Sampling Plans

• Probability sample

– Systematic sampling

• The selection of study participants such that every Xth person or element in a sampling frame or list is chosen

• Population is divided by the size of desired sample to obtain a sampling interval

• Sampling interval is the standard distance between the selected elements

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Sampling Plans

• Sample Size (Quantitative Studies)

– Sample size

• The number of participants in a sample

• Use the largest sample possible

• The larger the sample, the more representative it is likely to be

• The larger the sample, the smaller the sampling error

• Large samples counter balance atypical values

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Critiquing the Sampling Plan

• Did the researcher adequately describe the sampling plan– Type of sampling used– The population under study– Number of participants– Main characteristics of participants– Number and characteristics of potential subjects

• Were good sampling decisions made

• Was the sample representative of the population

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Critiquing the Sampling Plan

• Response rates– The number of people participating in a study relative

to the number of people sampled

• Nonresponse bias– Differences between participants and those who

declined to participate– A bias that can result when a nonrandom subset of

people invited to participate in a study fail to do so

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• Sampling in Qualitative Studies

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Sampling in Qualitative Studies

• Uses small samples

• Non-random samples

• Sample design is emergent

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Sampling in Qualitative Studies

• Types of Qualitative Sampling

– Convenience sampling (volunteer sample)– Snowball sampling– Purposive sampling (theoretical sampling, purposeful

sampling)• Researcher selects sample based on information needs

which emerged from earlier findings

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Sampling in Qualitative Studies

• Sample Size– Sample size is based on informational needs– Data saturation is sought

• Sampling to the point at which no new information is obtained and redundancy is achieved

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Sampling in Qualitative Studies

• Evaluating Sampling Plans Based on:– Adequacy

• Sufficiency and quality of the data the sample yielded

– Appropriateness• Using the best informants for the sample, those who will

provide the best information

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Reference

• Loiselle, C. G., Profetto-McGrath, J., Polit, D. F., & Beck, C. T. (2011). Canadian essentials of nursing research. (Third Edition). Philadelphia: Lippincott, Williams & Wilkins.