SELECTION OF A SAMPLE

By

Dr.Shazia Zamir

Contents

SELECTION OF A SAMPLE

- The Simple Random Sample

- The Systematic Sample

- The Stratified Random Sample

- The Area or Cluster Sample

TYPES OF SAMPLE

- Probability sampling

- Non-probability sampling

PopulationIncluding all people or item with the characteristic one wishes

to understand.

Area where the results of the study are generalized.

Because there is very rarely enough time or money to gather

information from everyone or everything in a population,

the goal becomes finding a representative sample (or

subset) of that population.

Examples of Population

All students of class X in Rawalpindi district.

All seventh grade students of IMCBs.

All the foreign students enrolled in NUML.

…..

SAMPLE

- A smaller group, which is selected from the

population to be included in the research.

- Sample has to be representative of characteristics of

the population.

- Results are drawn from the sample and

generalized to the entire population.

- Sample size depends upon the requirements of the

research.

Sampling Design Process

Define Population

Determine Sampling Frame

Determine Sampling Procedure

Probability Sampling

Simple Random

Sampling

Stratified Sampling

Cluster Sampling

Non-Probability Sampling

Convenience

Purposive

Quota

Snow ball

Determine Appropriate

Sample Size

Execute Sampling

Design

Why Sampling?

Time saving

Cost saving

Examine in greater detail

More scientific

Representative

Accessible

….

Major Sampling Techniques

Two types of major sampling techniques

1. Probability sampling

2. Non-probability sampling

Probability Sampling

Every individual in the population has greater than zerochance of selection.

Ensures a more representative sample

Types of probability sampling

1. Simple random sampling

2.Stratified sampling

3. Cluster sampling

Simple Random Sampling

Define population

Develop sampling

frame

Assign each unit a

number

Randomly select the required

amount of random

numbers

Systematically select random numbers until it meets the sample size

requirements

Every member of population

has equal probability of selection.

Used in case of homogeneous population

Use any of the techniques for random

selection such as draw, use of random number table

Stratified Sampling

Where population embraces a number of distinct

categories, the frame can be organized into separate

"strata." Each stratum is then sampled as an independent

sub-population, out of which individual elements can be

randomly selected. Every unit in a stratum has same chance

of being selected.

Using same sampling fraction for all strata ensures

proportionate representation in the sample.

Adequate representation of minority subgroups of interest

can be ensured by stratification & varying sampling

fraction between strata as required.

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Stratified Random Sampling

-Used in case of heterogeneous population

-Population split up into different strata on the bases of theirpeculiar features of interest to researcher.

-Then random sample picked proportionately from eachstratum. e,g gender, age, ethnic group etc

Cluster Sampling

Selecting a sample based on specific, naturally occurring

groups (clusters) within a population.

Example: randomly selecting 10 public sector universities

from a list of all public sector universities in Pakistan.

Systematic Sampling

List the population with serial numbers.

-Determine the sample size.

-Divide the population by SS to find out Kth case.

-Randomly start taking every kth case.

-Go on picking every kth case.

-Continue even afresh till you complete the sample size.

Non-Probability Sampling

- Where in every member doesn’t have equal chanceof selection.

1. Purposive Sampling

Look for the population of your interest e.g. inselecting a school to perform experiment inmethods of teaching.

2. Quota Sampling

Just as in case of SRS, except that you purposefullyselect certain cases/units to give representation tocover the quota.

3. Snowball Sampling

Ask the sample to identify another one and go on tillthe completion of sample size.

4. Convenience Sampling

Useful for pilot-testing

Pick up the cases easily available.

Selection of whichever individuals are easiest to reach

It is done at the “convenience” of the researcher