Selecting the Population

8/2/2019 Selecting the Population

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Selecting the Population

and Sample

Welcome

A. Ravono.


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Population

Whole area of interest

Living and non- living things/ events


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Types of population

2 Types

Finite

Infinite


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Sampling

Process by which the sample is selectedfrom the population

A portion of the whole group under study

Vital part of the research process


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Advantages of Sampling

Cheaper, Faster, Easier

Maybe the only part where destructivetesting is used- (extinct population)

Accurate than census


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Errors in Sampling

Not totally accurate

Non- sampling errors


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How to take a Sample?

Take a large sample to ensurerepresentations

Avoid non- sampling errors

Ensure reliability


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Types of Samples

Probability Sampling

Simple Random

Stratified Random

Systematic

Cluster

Non- Probability

Accidental orConvenience

Snowball

Purposive or Judgment

Expert

Quota


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Probability Samples (Random)2 principles

1. Each unit is a member of the population-has a probability of being selected in thesample that can be calculated

2. Selection of unit must be random(determined by chance)


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Advantages of probability sample

Possible to assess the accuracy of sampleestimates


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1. Simple Random Sample

Simplest form of probability sampling

Has equal chance of being chosen

Has equal chance of arising


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Using a table of random number

Have a complete list of whole population(sampling frame)

Assign a number to each population item

Choose the sample by means of randomtable.


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2. Systematic Sample

Work systematically through population list

No natural ordering or periodicity

Ordered lists- bias the results Stratification- widely used in large scale

survey


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3. Stratified Random Sampling

Stratum- sub population

Strata- 2 or more homogeneous subpopulation

- select population and determine relevantstrata

- sample number of people in eachstratum


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4. Cluster Sampling

Divide a large area into sub- areas whichare randomly selected- clusters

Interview all in a cluster or only a samplein a selected cluster (multi- stagesampling)


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Non- Probability Sampling

Used when impossible to evaluate theprobability of each population unit beingincluded in the sample


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1. Accidental Sampling

Sample of Convenience

Composed of easily available people orvolunteers

Not accurate

Includes snowball sampling


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2. Quota Sampling

Discretion of interviewers

Attempts to include a cross- section of thepopulation

Depends on the judgment of theinterviewers


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3. Purposive Sample

Units selected by researcher to berepresented under study

Depends on researchers judgment


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Pilot Study

Definition: small scale practice run

Not used by new researchers/ undergrad

Used to understand strengths andweaknesses of larger projects- (design,sample size, data collection instrument)

Helps in removing problems in advance Used for approval of funds.


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Questions?????

Documents

Selecting the Population