Download - 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?????