brm lecture 11

7/31/2019 BRM Lecture 11

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Sampling

Business Research Methods

Lecture: 13

Zain-Ul-Abideen


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Sampling

Sampling: The process of selecting a sufficientnumber

of elements from the population, so that results from

analyzing the sample are generalizable to the population.

OR

The basic idea of sampling is that by selecting some

of the elements in population, we draw conclusions

about the entire population.

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Population refers to the totality of people, events, things of

interest or objects (which may be individuals, households,

organizations, countries etc.) that the researcher wishes to

investigate. E.g. All office workers in the firm compose apopulation of interest; all 4,000 files define a population of

interest.

An elementis a single member of the population.

Element is the unit of study; it may be a person or may be

something else.

E.g.: Each staff member questioned about an optimal

promotional strategy is a population element. Each advertising account analyzed is an element of an account

population

Each ad is an element of a population of advertisements

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Census

A census is a count of all the elements in a population;

If 4,000 files define the population, a census would obtain

information from every one of them.

Sample: A subset of the population selected to investigate the

properties of the population. Because populations are often

extremely large, or even infinite, it is usually impossible for costand practical reasons to take measurements on every element

of the population. For this reason, more often, we draw a sample

and generalize from the properties of the sample to the broader

population. Sampling unit:The element or set of elements that is available for

selection in some stage of the sampling process.

A subjectis a single member of the sample, just as an element is

a single member of the population.5


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Statistics versus Parameters

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Questions

You wish to study the care arrangements of at

government hospitals in Islamabad and Rwp.

Find out the opinions of workers in a factory on changedworking arrangements

Measuring students satisfaction level about teaching in

the MBA/BBA/BS/MS programs

Find out the changing attitude of Pakistanis towards

immigration to Australia, NZ, USA, UK.


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Advantages of Sampling

Sometimes there is a need for sampling. Suppose we want to inspect the

eggs, the bullets, the missiles and the tires of some firm. The study may

be such that the objects are destroyed during the process of inspection.Sampling plays a key role in this process.

Sampling saves money as it is much cheaper to collect the desired

information from a small sample than from the whole population.

Sampling saves a lot of time and energy as the needed data are collectedand processed much faster than census information.

Sampling makes it possible to obtain more detailed information from each

unit of the sample as collecting data from a few units of the population.

Sampling has much smaller non-response, following up of which ismuch easier.

The most important advantage of sampling is that it provides a valid

measure of reliability for the sample estimates.

Sample data is also used to check the accuracy of the census data. 10


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The Sampling Process

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Major steps in sampling: Define the population

(elements, geographic boundaries, and time)

Determine the sample frame Determine the sampling design

Determine the appropriate sample size

Execute the sampling process


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Sampling Techniques

Probability Sampling Simple Random Sampling

Systematic Sampling

Stratified Random Sampling Cluster Sampling

Nonprobability Sampling

Convenience Sampling Judgment Sampling

Quota Sampling

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Simple random sampling

(all members have equal chance of being selected)O O O O O

X O O O X

O O O O O

O O X O O

O O O X O


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Systematic Sampling

Procedure

Each nth element, starting with random choice of an

element between 1 and n

Characteristics

Easier than simple random sampling

Systematic biases when elements are not randomlylisted

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Systematic sampling(systematic sampling involves selecting every nth case within a defined

population)

O O O O X

O O O O X

O O O O X

O O O O X

O O O O X


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Cluster Sampling

Procedure Divide of population in clusters

Random selection of clusters

Include all elements from selected clusters

Characteristics

Intercluster homogeneity

Intracluster heterogeneity Easy and cost efficient

Low correspondence with reality

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Cluster sampling (cluster sampling involves surveyingwhole clusters of the population selected through a defined

random sampling strategy.) O

O

O

O

O

O


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Stratified Sampling

Procedure Divide of population in strata

Include all strata

Random selection of elements from strata

Proportionate

Disproportionate

Characteristics

Interstrata heterogeneity

Intrastratum homogeneity

Includes all relevant subpopulations

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Example

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Stratified random sampling


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Stratified random sampling(Dividing your population into various subgroups and then taking a simple

random sample within each.)O X O O O

O O O X O

O O O O X

X O O O O

O X O O O

Overview


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Overview


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Overview


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Cl t Li ti U it d El t U it


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Clusters, Listing Units and Elementary Units

CLUSTER LISTING UNIT ELEMENTARY UNIT

City Block Household Person

County Hospital Patient

School Class Room Student

Page of Text Line of Text Word

Week Day Hour

http://www.gsb.tt/academic/uploads/ResearchMethodsSession63.ppt#309,15,Clusters, Listing Units and Elementary Units


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Tradeoff between precision and confidence

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We can increase both precision (how close our estimate is to the

true population chartacteristics) and confidence ( how certain weare that our estimate will really hold true for the population) to

increasing the sample size.


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Sample size: guidelines

In general: 30 < n < 500

Categories: 30 per subcategory

Multivariate: 10 x number of vars

Experiments: 15 to 20 per condition

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S l Si f Gi


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Sample Size for a Given

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Useful website for sample size formula

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http://www.isixsigma.com/library/content/c000709a.asp

http://www.surveysystem.com/sscalc.htm

http://www.stats.gla.ac.uk/steps/glossary/sampling.html#clustsamp

http://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspx
http://www.isixsigma.com/library/content/c000709a.asphttp://www.surveysystem.com/sscalc.htmhttp://www.stats.gla.ac.uk/steps/glossary/sampling.htmlhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.gallup.com/video/111154/Slight-Downtick-Economic-Negativity.aspxhttp://www.stats.gla.ac.uk/steps/glossary/sampling.htmlhttp://www.surveysystem.com/sscalc.htmhttp://www.isixsigma.com/library/content/c000709a.asp


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Thanks to Allah

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brm lecture 11

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