SAMPLINGSAMPLING

DR. SEEMA MUMTAZDR. SEEMA MUMTAZ

SAMPLING PROCEDURE SAMPLING PROCEDURE HAS TO FOLLOW THREE HAS TO FOLLOW THREE FUNDAMENTALSFUNDAMENTALS

• IT IS REPRESENTATIVEIT IS REPRESENTATIVE

• IS LARGE ENOUGHIS LARGE ENOUGH

• THE SELECTED ELEMENTS THE SELECTED ELEMENTS HAVE BEEN PROPERLY HAVE BEEN PROPERLY APPROACHED, INCLUDED & APPROACHED, INCLUDED & INTERVIEWED INTERVIEWED

SAMPLINGSAMPLINGSAMPLE IS A SUB SET OF SAMPLE IS A SUB SET OF

TOTAL POPULATION.TOTAL POPULATION.IT HAS ALL INHERRANT IT HAS ALL INHERRANT

QUALITIES OF QUALITIES OF POPULATION.POPULATION.

WE CAN DERIVE WE CAN DERIVE INFERENCES ABOUT THE INFERENCES ABOUT THE

POPULATION FROM POPULATION FROM SAMPLE.SAMPLE.

SAMLE HAS TO BE A SAMLE HAS TO BE A TRUE REPRESENTATIVE OF TRUE REPRESENTATIVE OF

WHOLE POPULATION.WHOLE POPULATION.

REQUISITE FOR A RELIABLE REQUISITE FOR A RELIABLE SAMPLE :SAMPLE :

• EFFICIENCYEFFICIENCY

• REPRESENTATIVENESSREPRESENTATIVENESS

• MEASURABILITYMEASURABILITY

• SIZESIZE

• COVERAGECOVERAGE

• GOAL ORIENTEDGOAL ORIENTED

• FEASIBILITYFEASIBILITY

• ECONOMY & COST ECONOMY & COST EFFICIENCYEFFICIENCY

Type of sampling Type of sampling procedures procedures

• Probability : Probability : each element has an equal each element has an equal chance of being included in the sample.chance of being included in the sample.

1. 1. simple random.simple random. 2. 2. Systematic.Systematic. 3. 3. Cluster.Cluster. 4. 4. Stratified.Stratified.

• Non- probability : Non- probability : There is no assuranceThere is no assurance that each element will have the same that each element will have the same

chance of being included in the sample.chance of being included in the sample. 1. 1. Accidental.Accidental. 2. 2. Purposive.Purposive. 3. 3. Quota Sampling.Quota Sampling.

Probability SamplingProbability Sampling

Simple Random:Simple Random: Each unit has the same chance of being selected, only decide by the law Each unit has the same chance of being selected, only decide by the law

of chance.of chance.

Procedure:Procedure:1. 1. Prepare a sampling frame-Prepare a sampling frame-list show all the units.list show all the units.2. 2. Decide on the number to be selected-Decide on the number to be selected-Sample sizeSample size. It is some what . It is some what

difficult but very critical.difficult but very critical. Three considerations in determination of sample size:Three considerations in determination of sample size: ~ variability in the population.~ variability in the population. ~ amount of accuracy desired.~ amount of accuracy desired. ~ resource available to the researcher.~ resource available to the researcher. ( table for determine sample size are available ).( table for determine sample size are available ). ~ type of study design. ~ type of study design. 3. 3. Selected require numbers of units, through.Selected require numbers of units, through. ~ drawing lots ( lottery method ) comes out of lotto, a game of chance.~ drawing lots ( lottery method ) comes out of lotto, a game of chance.

Probability SamplingProbability SamplingSystematic: Systematic: A pre-determined system is followed.A pre-determined system is followed.

Procedure:Procedure: 1. 1. Have the total numbers of units in the population.Have the total numbers of units in the population. 2. 2. Decide the sample size.Decide the sample size. 3. 3. Calculate the sampling ratio i.e. sample of 100 out Calculate the sampling ratio i.e. sample of 100 out

of 1000= 1:10of 1000= 1:10 370 out of 1600= 1:4.3 round it to the nearest whole 370 out of 1600= 1:4.3 round it to the nearest whole

numbers and it will become 1:4.numbers and it will become 1:4. 4. 4. Select randomly the first unit and interview every Select randomly the first unit and interview every

fourth unit that is 1:4.fourth unit that is 1:4.

Probability Sampling Probability Sampling

Cluster Sampling:Cluster Sampling: Selection is made of clusters of Selection is made of clusters of

groups such as mohallas, groups such as mohallas, apartment, buildings, villages, apartment, buildings, villages, housing unit etc.housing unit etc.

When a list of the entire area is When a list of the entire area is not available and is not not available and is not physically possible to visit the physically possible to visit the entire area, one can divide the entire area, one can divide the area into several equal size area into several equal size clusters.clusters.

Probability Sampling Probability Sampling

Stratified Sampling:Stratified Sampling: This is done to compare different strata.This is done to compare different strata.

Procedure:Procedure: 1. 1. Population is first divided into subgroups Population is first divided into subgroups

(Strata) according to one or more (Strata) according to one or more characteristics e.g.; age, sex, economic characteristics e.g.; age, sex, economic status, religion etc.status, religion etc.

2. 2. Follow a random or systematic procedure Follow a random or systematic procedure according to the available information. according to the available information.

Non-Probability Non-Probability SamplingSampling

Accidental ( convenience ) Sampling:Accidental ( convenience ) Sampling:

* Sample is selected in a haphazard fashion.* Sample is selected in a haphazard fashion.

* Sample selected is unlikely to be * Sample selected is unlikely to be representative.representative.

* The purpose of such sampling is usually less * The purpose of such sampling is usually less cost, convenience etc.cost, convenience etc.

e.g. First ten patient in clinic,e.g. First ten patient in clinic,

Students in library. Students in library.

NON-PROBABILITY NON-PROBABILITY SAMPLING SAMPLING

• PURPOSIVE: PURPOSIVE: Sampling done Sampling done on the basis of some pre on the basis of some pre determined idea (clinical determined idea (clinical knowledge etc. ).The result of knowledge etc. ).The result of such a sample can not be such a sample can not be generalized. Example:generalized. Example:

• Samples from different age Samples from different age groups.groups.

• Sample of all hypertensive. Sample of all hypertensive.

NON-PROBABILITY NON-PROBABILITY SAMPLING SAMPLING

• QUOTA SAMPLING :QUOTA SAMPLING : The strata of The strata of the population are identified & the the population are identified & the researchers determine the researchers determine the proportion of elements needed proportion of elements needed from the various segments, eg:from the various segments, eg:

• If in a population of students 40% If in a population of students 40% are female & 60% are males , then are female & 60% are males , then the researcher may decide for a the researcher may decide for a sample size 50 , to select 20 sample size 50 , to select 20 females & 30 males .then he can females & 30 males .then he can use any type of probability or non-use any type of probability or non-probability sampling procedures probability sampling procedures within the strata.within the strata.

REQUISITE OF A RELIABLE REQUISITE OF A RELIABLE SAMPLE SAMPLE

• EFFICIENCYEFFICIENCY

• REPRESENTATIVENESS REPRESENTATIVENESS

• MEASURABILITYMEASURABILITY

• SIZESIZE

• COVERAGECOVERAGE

• GOAL ORIENTEDGOAL ORIENTED

• FEASIBILITYFEASIBILITY

• COST- EFFECTIVENESSCOST- EFFECTIVENESS