sampling by dr najeeb memon
DESCRIPTION
sampling by Dr: Najeeb Memon LUMHS, Jamshoro Pakistan email [email protected]TRANSCRIPT
By: By:
Assist: Prof:Assist: Prof:Faculty of Community Medicine & Public Health Faculty of Community Medicine & Public Health SciencesSciencesLiaquat University of Medical & Health Sciences Liaquat University of Medical & Health Sciences (LUMHS)(LUMHS) Jamshoro, Sind, Jamshoro, Sind, PakistanPakistan e maile mail [email protected]@gmail.com
LIAQUAT
MEDICAL
UNIVERSITY
POPULATIONPOPULATION The whole group of the individuals on which the results of the study are to be generalized is known as Population. A
SAMPLE small group of individuals chosen for
the study is known as Sample.
The entire group of people of interest from whom the researcher needs to obtain information.
The selection of a subset of the population
Sampling FrameSampling FrameListing of all the members of the
universe from which the sample is to be taken , is known as Sampling Frame.
Sampling frame should be carefully developed as it affects the results drawn.
Listing of population from which a sample is chosen
WHY SAMPLING IS WHY SAMPLING IS NECESSARY?NECESSARY?
Convenient. Economic. Time saving. Less resources are required. Easy to interpret the data.
TYPES OF SAMPLINGTYPES OF SAMPLING
• A. Probability Sampling MethodsProbability Sampling Methods
• B. Non Probability Sampling MethodsB. Non Probability Sampling Methods
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SAMPLES TECHNIQUESSAMPLES TECHNIQUESA. A. Probability Sampling MethodsProbability Sampling Methods
Simple RandomSimple RandomSimple RandomSimple Random
systemic systemic samplingsamplingsystemic systemic samplingsampling
StratifiedStratifiedStratifiedStratified
ClusterClusterClusterCluster
MultistageMultistageMultistageMultistage
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B. Non probability B. Non probability Sampling Methods Sampling Methods
ConvenienceConvenienceConvenienceConvenience
JudgmentJudgmentJudgmentJudgment
QuotaQuotaQuotaQuota
SnowballSnowballSnowballSnowball
Each element in the population will have an equal chance of being included in the sample.
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1. Simple Random 1. Simple Random samplingsampling
1. Simple Random 1. Simple Random samplingsampling
No: is given to each of units (Persons/ houses)Table of random Nos: is usedSamples selected haphazardly (Each has equal chance of being selected)
N = 100 population size, n = 8 sample size
OR Lottery Method
E.g:- In Electro roll / Census
Simple random sampling
Selecting unit/ person Selecting unit/ person after a regular after a regular intervalinterval
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2. 2. systemic samplingsystemic sampling2. 2. systemic samplingsystemic sampling
Systematic sampling
3. Stratified3. Stratified3. Stratified3. Stratified
It involves division of population in to smaller groups (homogenous subgroups)
known as “Strata”
•Then Simple random sampling OR systematic sampling is applied with each stratum.
Example 2 Population is divided on the basis of characteristic of interest in the population E.g. male & female may have different consumption patterns.
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Here we select a simple random sample of groups such as a certain number of city blocks & then select a person each from each block.
This technique is more economical than the random selection of persons through out the city.
4. Cluster4. Cluster4. Cluster4. Cluster
Cluster samplingCluster sampling
Section 4
Section 5
Section 3
Section 2Section 1
• Sampling at different stages/ Levels• 1. National level• 2. Provincial level• 3. District level
5. Multistage5. Multistage5. Multistage5. Multistage
A sample selected in stages, beginning with the most unspecific level (such as regions) and ending with the most specific (such as houses on selected city blocks).
Non probability MethodsNon probability Methods
NON PROBABILITY NON PROBABILITY SAMPLINGSAMPLING
• Convenient
• Economic
BUT
RESULTS CAN NOT BE GENERALIZED
THEREFORE
Some probability factor is needed to be imposed to ensure a degree of representation in the sample.
A convenience sample is a sample where the patients are selected, in part or in whole, at the convenience of the researcher.
Example: Male on street interviews Teacher uses students
1. Convenience /Accidental/Incidental 1. Convenience /Accidental/Incidental samplingsampling
1. Convenience /Accidental/Incidental 1. Convenience /Accidental/Incidental samplingsampling
Involves the use of the most convenient & readily available subjects for sample.
The researcher chooses the sample based on who they think would be appropriate for the study.
2. Judgment /Purposive2. Judgment /Purposive2. Judgment /Purposive2. Judgment /Purposive
3. Quota3. Quota3. Quota3. Quota
if you are a researcher conducting a national quota sample, you might need to know what proportion of the population is male(40) and what proportion is female (60) as well as what proportions of each gender fall into different age categories, race or ethnic categories, educational categories, etc.
Researcher selects people according to some fixed quota.OR Keep going until the sample size is reached
Male 40
Female 60
Total (Fixed quota) 100
Researcher collects data on the few members of the target population he or she can locate, then asks those individuals to provide information needed to locate other members of that population whom they know.
4. Snowball4. Snowball4. Snowball4. Snowball
Get sampled people to nominate others
Sampling ErrorsSampling Errors A. Size: sample size should be large as possible
depends upon feasibility (time, person, importance of data)
B. subject variation: Sometimes observation may be changed on
different times (B.P at different times of a day)
C. Observer Variation: When 2 or more persons observe same data
(No same value) E.g:- Taking Blood Pressure reading
D. Technical Fault: By Instrument
E. Incomplete Coverage: If 10 out of 100 are non cooperative (i,e 90
or 10 Non co operative)
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