sampling & sampling methods

8/8/2019 Sampling & Sampling Methods

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Sample Designs and Sampling

Procedures


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Sampling The process ofsampling involves using a

small number of items or parts of the

population to make conclusions regarding

the whole population. The purpose of

sampling is to estimate some unknown

characteristic of the population.


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Sampling Terminology Sample

Population or universe Population element

Census


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Sample Subset of a larger population


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Population Any complete group that share some

common characteristics

People

Sales territories

Stores


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Census Investigation of all individual elements that

make up a population


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WHY SAMPLE? PRAGMATIC REASONS: Applied

business research projects usually havebudget and time constraints.

It would not be possible to contact the

whole population within a short period oftime.

Sampling cuts costs, reduces manpower

requirements, and gathers vital information

quickly.


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ACCURATE AND RELIABLE RESULTS:properly selected samples are sufficiently accurate in most

cases. Even when the populations have considerable

heterogeneity, large samples provide data of sufficient

precision to make most decisions.

A sample may be more accurate than a census. In a censusof a large population, there is greater likelihood of non-

sampling errors. In a field survey a small, well-trained,

closely supervised group may do a more careful and

accurate job of collecting information than a large group of

nonprofessional interviewers trying to contact everyone.


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DESTRUCTION OF TEST UNITS: Many

research projects, especially those in quality

control testing, require the destruction of the

items being tested.


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Define the target population

Select a sampling frame

Conduct fieldwork

Determine if a probability or nonprobability

sampling method will be chosen

Plan procedure

for selecting sampling units

Determine sample size

Select actual sampling units

Stages in the

Selection

of a Sample


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Target Population TOWHOM DOWEWANT TO TALK

It is the complete group of specific

population elements relevant to the research

project. E.g. in Industrial Buyer Behaviour

Relevant population

Operationally define


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Sampling Frame A list of elements from which the sample

may be drawn

Also called theWorking population Sampling frame error occurs when certain

elements are excluded or when the entire

population is not accurately represented in

the sample frame. It is possible for elements

to be either over- or underrepresented in a

sampling frame.


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Sampling Units

The sampling unit is a singles element or group of

elements subject to selection in the sample. For example, if

an airline wishes to sample passengers, every 25th name

on a complete list of passengers may be taken. In this case

the sampling unit would be the same as the element.

Primary Sampling Units (PSU)

Secondary Sampling Units

Tertiary Sampling Units


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Two Major Categories of Sampling

Probability sampling Known, nonzero probability for every element eachmember of the population has an equal probability

of being selected.

Non-probability sampling

Probability of selecting any particular member is

unknown

no appropriate statistical techniques for measuringrandom sampling error from a nonprobability

sample. Thus, projecting the data beyond the sample

is inappropriate.


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Nonprobability Sampling Convenience

Judgment

Quota

Snowball


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Probability Sampling Simple random sample

Systematic sample

Stratified sample

Cluster sample

Multistage area sample


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Convenience Sampling

Also called haphazard or accidental

sampling

The sampling procedure of obtaining the

people or units that are most conveniently

available

Best suited for exploratory research


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Judgment Sampling

Also called purposive sampling An experienced individual selects the

sample based on his or her judgment about

some appropriate characteristics required ofthe sample member

Especially used to forecast election results


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Quota Sampling Ensures that the various subgroups in a

population are represented on pertinent sample

characteristics to the exact extent that the

investigators desire

The interviewer has a quota to achieve Respondents are selected according to a

convenience sampling procedure rather than on a

probability basis, the haphazard selection of

subjects may introduce bias. Quota samples havethe tendency to include people who are easily

found, willing to be interviewed, and middle class.

It should not be confused with stratified sampling.


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Snowball Sampling A variety of procedures

Initial respondents are selected by

probability methods

Additional respondents are obtained from

information provided by the initial

respondents

Used to locate members of rare populations

by referrals.


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Simple Random Sampling A sampling procedure that ensures that each

element in the population will have an equal

chance of being included in the sample

Random numbers table can be used in large

population


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Systematic Sampling A simple process

Every nth name from the list will be drawn

Sampling Interval = population size/sample

size

The problem ofperiodicity occurs if a list

has a systematic pattern, that is, if it is not

random in character.


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Stratified Sampling

Probability sample Subsamples are drawn within different strata

Each stratum is more or less equal on some characteristic

Do not confuse with quota sample

First a variable is identified for basis of stratification. The variable chosen should increase the homogeneity

within each stratum and increase the heterogeneity

between strata. The stratification variable is usually a

categorical variable or one easily converted into categories,

that is, subgroups.

Next, for each separate subgroup or strata, a list of

population elements must be obtained


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Cluster Sampling

A population is divided into clusters usingnaturally occurring geographic or other

boundaries. The clusters are randomly selected

and a sample is collected by randomly selecting

from each cluster. The purpose of cluster sampling is to sample

economically while retaining the characteristics of

a probability sample.

The primary sampling unit is no longer theindividual element in the population

The primary sampling unit is a larger cluster of

elements located in proximity to one another


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Cluster

Population divided into manysubgroups

Each subgroup has fewelements in it

Subgroups are selectedaccording to some criterion ofease

Heterogeneity within subgroups

Homogeneity betweensubgroups

Random choice of subgroups

Stratified

Population divided into few subgroups

Each group has many elements in

it.

Subgroups are selected according

to some criterion that is related to

the variables under study

Homogeneity within subgroups

Heterogeneity between subgroups

Choice of elements from withineach subgroup


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What is the

Appropriate Sample Design? Degree of accuracy

Resources

Time

Advanced knowledge of the population

National versus local Need for statistical analysis

sampling & sampling methods

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