BY: SHOBHA RAWATJR MFSC.

(FRM) 24-12-2014

INTRODUCTION

NON PROBABILITY SAMPLING

TYPES OF NON PROBABILITY SAMPLING

CONVENIENCE SAMPLING

VOLUNTEER SAMPLING

24-12-2014

INTRODUCTION

WHAT IS SAMPLE?

*A sample is “a smaller (but hopefully representative) collection of units from a population used to determine truths about that population”.

(Field, 2005).

24-12-2014

SAMPLE

TARGET POPULATION

STUDY POPULATION

SAMPLE

24-12-2014

Cont’d…

SAMPLING

Art Of Obtaining

Samples From The Population

Under Consideration.

24-12-2014

Cont’d…

SAMPLING

i.e

Data Under Observation

24-12-2014

Cont’d…

EXAMPLES OF SAMPLING

Height & Weight data of fishes of a Monoculture Pond

System.

Water sample from any natural water body for productivity

analysis.

24-12-2014

Cont’d…

*Lenth & Weight

Measurement

24-12-2014

Cont’d…

24-12-2014

Cont’d…

NON PROBABILITY

SAMPLING24-12-2014

NON-PROBABILITY SAMPLING

Sampling Technique that do not let us know in advance the likelihood of selecting for the

sample each element or case from a population.

OR

Any sampling method where some elements of population have no chance of selection

(referred to as 'out of coverage'/'undercovered')

OR

where the probability of selection can't be accurately determined.

24-12-2014

Cont’d…

ASSUMPTION

There is an even

distribution of characteristics

within the population.

24-12-2014

Cont’d…

Assumption

Arbitrarily Sampling

Precision

24-12-2014

Cont’d…

EXAMPLE

* We visit every household in a given street, and interview the first person to answer the door.

* In any household with more than one occupant, this is a nonprobability sample, because some people are more likely to answer the door .

*(e.g.) An unemployed person who spends most of their time at home is more likely to answer than an employed housemate who might be at work when the interviewer calls)

* It's not practical to calculate these probabilities.

24-12-2014

Cont’d…

Non-probability SamplingOverview

Recruit people that are easy to reach or belong to a group that the assessor is interested

People could be best/worst/typical/unique case

Criteria must be established on how people are recruited and how characteristics are assessed

24-12-2014

Cont’d…

When to Use Non-

Probability Sampling??

*When demonstrating that a particular trait

exists in the population.

*Researcher aims to do a qualitative, pilot

or exploratory study.

*When randomization is impossible like

when the population is almost limitless.

24-12-2014

Cont’d…

When to Use Non-

Probability

Sampling??

*When the researcher has limited budget, time and

workforce.

*The research does not aim to generate results that

will be used to create generalizations pertaining to

the entire population.

*Primarily used for pilot testing

24-12-2014

Cont’d…

AREAS OF INTEREST

Psychology Medicine Sociology

Exploratory &

Qualitative research

Hard-to-discover

populations

24-12-2014

Cont’d…

CLASSIFICATION

Convenience

Or

Haphazard Sampling

Volunteer Sampling

Judgement Sampling

Quota Sampling

24-12-2014

Cont’d…

CONVENIENCE

SAMPLING24-12-2014

CONVENIENCE

SAMPLING

*As Haphazard or Accidental

sampling.

*Sample units are only selected if

they can be accessed easily and

conveniently.

24-12-2014

Cont’d…

Fish sampling for

behaviorological study24-12-2014

Cont’d…

CHARACTERSTICS

It is not normally

representative of the target population

The population is

homogeneous.

24-12-2014

Cont’d…

AREAS OF INTEREST

Experiments

Exploratory And

Qualitative Research

24-12-2014

Cont’d…

ADVANTAGES

Easy to use

Less time & cost required

Easy acess to data &

information

24-12-2014

Cont’d…

LIMITATIONS

presence of bias

Misleading information

Sample not the representative of population being studied

24-12-2014

Cont’d…

EXAMPLE

*A scientist could use this method to determine

whether a lake is polluted.

*Assuming that the lake water is well-mixed,

any sample would yield similar information.

*A scientist could safely draw water anywhere

on the lake without fretting about whether or

not the sample is representative.

24-12-2014

Cont’d…

OTHER EXAMPLES

*Examples of convenience sampling includes:

*The female moviegoers sitting in the first row

of a movie theatre.

*The first 100 customers to enter a department

store.

*The first three callers in a radio contest.

24-12-2014

Cont’d…

*VOLUNTEER SAMPLING

24-12-2014

VOLUNTEER SAMPLING

* This type of sampling occurs when

people volunteer their services for

the study.

* Samples are drawn through.

* Volunteer sample typically biased

24-12-2014

Cont’d…

SOURCE OF

INFORMATION

Advertising

Requesting people to

participate for study.

24-12-2014

Cont’d…

ADVANTAGES

The people who care strongly

enough about the subject one way or

another tend to respond.

Doesn't require a lot of screening

You can just put up an

advertisement and use whoever responds.

Useful when respondents

are difficult to recruit by any other means.

24-12-2014

Cont’d…

LIMITATIONS

Strong biases

Sampling of voluntary participants only

Silent majority does not typically respond.

May clash with the researcher's aims.

24-12-2014

Cont’d…

EXAMPLES

* In psychological experiments or pharmaceutical trials (drug testing),

* for example, it would be difficult and unethical to enlist random participants from the general public.

* In these instances, the sample is taken from a group of volunteers. Sometimes, the researcher offers payment to entice respondents.

* In exchange, the volunteers accept the possibility of a lengthy, demanding or sometimes unpleasant process.

24-12-2014

Cont’d…

EXAMPLES

24-12-2014

Cont’d…

EXAMPLES

*Use of call-in polls by television and radio

media

24-12-2014

Cont’d…

24-12-2014