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Population and Sampling 

 After the completion of theunit-VII student will be able to 

Define the concepts related to sampling theory.

Distinguish between probability and non- probability sampling.

Identify several types of samples and describe their main characteristics.

Describe approaches to determining samplesize.

Evaluate the sampling approach in aquantitative research study.

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Some important concepts

± Sampling:

Selecting a group of people, events, behaviors,or other elements with which to conduct astudy.

± Sample:

The selected group of people (or elements) from which data are collected for a study 

± Population:

The entire set of individuals (or elements) who (that) met the sampling criteria. Also called target population

± Accessible Population:The portion of the target population to which theresearcher has reasonable access.

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Representativeness

The sample, the accessible population, and 

the target population are alike in as many 

ways as possible.

±

Needs to evaluate: Setting:

Characteristics of the subjects:

± age

± Gender 

± ethnicity ± income

± education

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Reasons for Drawing a

Sample

Less time consuming than a census

Less costly to administer than a census

Less cumbersome and more practical to 

administer than a census of the targeted 

 population

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Types of Samples Used 

Nonprobability Sample

± Items included are chosen without regard 

to their probability of occurrence

Probability Sample

± Items in the sample are chosen on the

basis of known probabilities

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Types of Sampling Used 

Quota

Sampling

Non-ProbabilitySampling

Convenience PurposivePurposive

Probability

Sampling

SimpleRandom

Systematic

Stratified

Cluster NetworkNetwork

(continued)

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Probability Sampling 

Items in the sample are chosen based on

known probabilities

Probability Samples

Simple

RandomSystematic Stratified Cluster 

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Simple Random Sampling 

Every individual or item from the frame

has an equal chance of being selected 

Selection may be with replacement or 

without replacement

Samples obtained from table of random 

numbers or computer random number 

generators

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Decide on sample size: n

Divide frame of N individuals into groups

of k individuals: k=N/n

Randomly select one individual from the1st group 

Select every k th individual thereafter 

Systematic Sampling 

N = 64

n = 8 

k = 8  First Group 

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Stratified Sampling

Divide population into two or more subgroups

(called strata) according to some common

characteristic 

 A simple random sample is selected from eachsubgroup, with sample sizes proportional to strata

sizes

Samples from subgroups are combined into one

Population

Divided into 

4 strata

Sample

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Cluster Sampling

Population is divided into several ³clusters,´ 

each representative of the population

 A simple random sample of clusters is selected 

± All items in the selected clusters can be used,

or items can be chosen from a cluster using 

another probability sampling technique

Population

divided into 

16 clusters. Randomly selected 

clusters for sample

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 Advantages and Disadvantages

Simple random sampling and systematic 

sampling 

± Simple to use

± May not be a good representation of the

 population¶s underlying characteristics

Stratified sampling 

± Ensures representation of individuals

across the entire population

Cluster sampling 

± More cost effective

± Less efficient (need larger sample to 

acquire the same level of precision)

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Factors Influencing Sample

Size

Effect Size

Type of Study Conducted 

Number of Variables

Data Analysis Techniques

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Power Analysis

Standard Power of 0.8

Level of Significance

alpha = .05, .01, .001

Effect Size

.2 Small; .5 Medium; .8 Large

Sample Size

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Thank You 

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Research Topic 

Group 1: Patient¶s satisfaction and nursing services

Group 2: White coat syndrome and blood pressure in8 to 10 years children

Group 3: Self medication practice in nurses

Group 4: Nurse¶s knowledge and practices about patient¶s medication

Group 5: What is future planning after competitionnursing course? 

Group 6: What is the difference in night and  morning shifting impact on nurse¶s life?