BRM_ch16

8/18/2019 BRM_ch16

http://slidepdf.com/reader/full/brmch16 1/35

Chapter 16:Sample Designs

andSampling Procedures

8/18/2019 BRM_ch16


Learning Outcomes

After studying we should be able to• !plain reasons for ta"ing a sample rather than a

complete census• Describe the process of identifying a target

population and selecting a sampling frame• Compare random sampling and systematic

#nonsampling$ errors• %dentify the types of nonprobability sampling&

including their ad'antages and disad'antages

• Summari(e the ad'antages and disad'antages ofthe 'arious types of probability samples• Discuss how to choose an appropriate sample

design& aswell as challenges for %nternet sampling

2

8/18/2019 BRM_ch16


Changing Poc"etboo" Problems for )oday*s +amilies

• %t is easy to as" people what theyconsider to be the most pressing,nancial problems they face-

– Low wages – .ising health care and housing costs

– )oo much debt

• /hen pressed about which ,nancialproblem is most important& someinteresting trends occur-

3

8/18/2019 BRM_ch16



• ach 0uarter& the allup Corporationde'elops a representati'e sample ofappro!imately 1&222 3-S- adults& aged 14and older-

• .esearchers can be 5 percent con,dentthat the responses of the sample arere7ecti'e of this national population& witha sampling error of less than 8 percent-

• 3sing telephone based inter'iews& theallup Corporation as"s the respondent todescribe 9the most important ,nancialproblem facing your family today-

4

8/18/2019 BRM_ch16



• )rends suggest that the most important,nancial problem facing families canoften change o'er time-

• %n ;224& energy and gas prices was theirmost important problem-

• %n ;225& health care costs wasmentioned-

• The use of large-scale representativesamples by the Gallup Corporationhelped reveal these interesting trends.

5

8/18/2019 BRM_ch16


Sampling )erminology

• A sample is a subset of a larger population- – )he purpose of sampling is to estimate an un"nown

characteristic of a population-

• A population is any complete group<for

e!ample& of people& sales territories& stores& orcollege students-

– Shares some common set of characteristics-

• Population element refers to an indi'idual

member of the population-• A census is an in'estigation of all the

indi'idual elements that ma"e up thepopulation-

6

8/18/2019 BRM_ch16


/hy Sample=

• Pragmatic .easons: – >udget and time constraints? for e!ample census

of past purchasers* reactions of +ord motors-

– Sampling cuts costs& reduces labor re0uirements&and gathers 'ital information 0uic"ly-

• Accurate and .eliable .esults: Sample may onoccasion be more accurate than a census sincenonsampling errors may increase duringbecause of the increased 'olume of wor"-

• Destruction of )est 3nits: specially those in0uality@control testing& re0uire the destructionof the items being tested- +or e!ample& testingbulbs& blood count-

7

8/18/2019 BRM_ch16


Stages in the Selection of aSample

8

8/18/2019 BRM_ch16



1- De,ning the )arget Population: – One sur'ey concerning organi(ational buyer

beha'ior incorrectly de,ned the population aspurchasing agents whereas industrial engineers

substantially aected buying decisions-;- )he Sampling +rame: A list of elements

from which a sample may be drawn? alsocalled wor"ing population-

– A list of all members of +inance AlumniAssociation-

– %n practice& almost e'ery list e!cludes somemembers of the population-

9

8/18/2019 BRM_ch16



– Sampling frame error: error that occurs whencertain sample elements are not listed or arenot accurately represented in a samplingframe-

• +or e!ample uni'ersity student e@mail directory- – Population elements can be either under@ or

o'errepresented in a sampling frame-

• +or e!ample& a sa'ings and loan de,ned its

population as all indi'iduals who had sa'ingsaccounts- Bowe'er& indi'iduals who hadmultiple accounts were o'errepresented in thesample-

10

8/18/2019 BRM_ch16



8- Sampling 3nits: A single element orgroup of elements subect to selectionin the sample-

– +or e!ample& if an airline wishes to sample

passengers& it may ta"e e'ery ;th name ona complete list of passengers-

– Primary Sampling 3nits #PS3$: A unitselected in the ,rst stage of sampling-

– Secondary Sampling 3nits: selected in thesecond stage of sampling-

– )ertiary Sampling 3nits: Selected in the thirdstage of sampling-

11

8/18/2019 BRM_ch16


.andom Sampling rror

• )he dierence between the sampleresults and the result of a censusconducted using identical procedures

• Statistical 7uctuation due to chance'ariations means the sampling units&e'en if properly selected according tosampling theory& may not perfectlyrepresent the population& but generallythey are reliable estimates-

• As sample si(e increases& randomsampling error decreases-

12

8/18/2019 BRM_ch16


Systematic rrors

• rrors result from nonsamplingfactors such as unrepresentati'esample results

• ot due to chance• Due to study design or imperfections

in e!ecution

13

8/18/2019 BRM_ch16


Less )han Perfectly .epresentati'eSamples

• Sample considering both errors produce less thanperfectly representati'e samples-

14

8/18/2019 BRM_ch16


)wo Eaor Categories of Sampling

• Probability sampling: – e'ery member of the population has a

"nown& non(ero probability of selection-

• onprobability sampling

– Probability of selecting any particularmember is un"nown-

– .esearchers rely hea'ily on personal

udgment- – o appropriate statistical techni0ues e!istfor measuring random sampling error froma nonprobability sample-

15

8/18/2019 BRM_ch16


onprobability Sampling

• Con'enience

• Fudgment

• Guota• Snowball

16

8/18/2019 BRM_ch16


Con'enience Sampling

• )he sampling procedure of obtaining thepeople or units that are most con'enientlya'ailable- – +or e!ample& tele'ision stations often present

person@on@the@street inter'iews that are

presumed to re7ect public opinion• 3sed to obtain a large number of completed

0uestionnaires 0uic"ly and economically-• 3sed when obtaining a sample through

other means is impractical- – Also called hapha(ard or accidental sampling

• Are best used for e!ploratory researchwhen additional research will subse0uentlybe conducted with a probability sample-

17

8/18/2019 BRM_ch16


Fudgment Sampling

• An e!perienced indi'idual selects thesample based on his or her udgmentabout some appropriate characteristicsre0uired of the sample member- – Also called purposi'e sampling-

– +or e!ample the consumer price inde! #CP%$is based on a udgment sample of mar"et@bas"et items& housing costs& and otherselected goods and ser'ices-

•.esearchers select samples that satisfytheir speci,c purposes& e'en if they arenot fully representati'e- – Fudgment sampling often is used in attempts

to forecast election results-

18

8/18/2019 BRM_ch16


Guota Sampling

• nsures that the 'arious subgroups in apopulation are represented on pertinentsample characteristics to the e!act e!tentthat the in'estigators desire-

• %t should not be confused with strati,edsampling since the inter'iewer has a 0uotato achie'e-

– +or e!ample& an inter'iewer in a particular citymay be assigned 122 inter'iews& 8 withowners of Sony )Hs& 82 with owners ofSamsung )Hs& 14 with owners of Panasonic )Hs& and the rest with owners of other brands-

19

8/18/2019 BRM_ch16


Guota Sampling

• Possible bias: – Guota samples tend to include people who

are easily found& willing to be inter'iewed&and middle class-

• Bowe'er oers speed of data collection&lower costs& and con'enience-

• Appropriate when the researcher "nowsthat a certain demographic group is more

li"ely to refuse to cooperate with a sur'ey- – +or instance& if older men are more li"ely torefuse& a higher 0uota can be set for thisgroup-

20

8/18/2019 BRM_ch16


Snowball Sampling

• %nitial respondents are selected by

probability methods Additionalrespondents are obtained from informationpro'ided by the initial respondents-

• 3sed to locate members of rare

populations by referrals- – Suppose a manufacturer of sports e0uipmentis considering mar"eting a mahogany cro0uetset for serious adult players-

• .educed sample si(es and costs are clear@

cut ad'antages of snowball sampling-• >ias is li"ely to enter into the study

because one is suggesting another-• Appropriate for focus groups-

21

8/18/2019 BRM_ch16


Probability Sampling

• Simple random sample• Systematic sample

• Strati,ed sample

• Cluster sample

• Eultistage area sample

22

8/18/2019 BRM_ch16


Simple .andom Sampling

• A sampling procedure that ensuresthat each element in the populationwill ha'e an e0ual chance of being

included in the sample-

• A table of random numbers can beused-

• /idely used-

23

8/18/2019 BRM_ch16


Systematic Sampling

• A starting point is selected by a randomprocess and then e'ery nth number on thelist is selected- – Suppose a researcher wants to ta"e a sample of

1&222 from a list of ;22&222 names- /ith

systematic sampling& e'ery ;22th name from thelist would be drawn-

• )he problem of periodicity #the tendency torecur at inter'als$ occurs if a list has a

systematic pattern<that is& if it is notrandom in character- – Collecting retail sales information e'ery se'enth

day would result in a distorted sample becausethere would be a systematic pattern-

24

8/18/2019 BRM_ch16


Strati,ed Sampling

•Characteristics of the samples arehomogenous within the group butheterogeneous across the group-

• Strata: Di'iding the population into subgroups-

• Stratum: ach subgroup is "nown as stratum-• Choosing strata on the basis of e!isting

information-

– +or e!ample& classifying retail outlets based

on annual sales 'olume-

• )hen a subsample is drawn using simplerandom sampling within each stratum-

25

8/18/2019 BRM_ch16


Strati,ed Sampling

• 3sed to obtain a more eIcient sample than a

simple random sampling-

– +or e!ample& data are collected both from urbanand rural customers-

• .andom sampling error will be reduced and

sample will accurately re7ect the population-

• %n a proportional strati,ed sample the number ofsampling units drawn from each stratum is inproportion to the relati'e population si(e of the

stratum-• %n a disproportional strati,ed sample the sample

si(e for each stratum is not allocated inproportion to the population si(e-

26

8/18/2019 BRM_ch16


Strati,ed Sampling

27

8/18/2019 BRM_ch16


Cluster Sampling

• )he primary sampling unit is not theindi'idual element in the population but alarge cluster of elements? clusters areselected randomly-

• conomical while retains the characteristicsof a probability sample- – Consider a researcher who must conduct ,'e

hundred personal inter'iews with consumersscattered throughout >angladesh- )ra'el costs

and time are li"ely to be enormous- %f the producte0ually appeals dwellers of Dha"a and Chittagongthen only one cluster can be inter'iewed-

• %deally a cluster should be heterogeneous-

28

8/18/2019 BRM_ch16


8/18/2019 BRM_ch16


30

8/18/2019 BRM_ch16


31

8/18/2019 BRM_ch16


/hat is the Appropriate SampleDesign=

• Degree of accuracy

• .esources

• )ime• Ad'anced "nowledge of the

population

• ational 'ersus local• eed for statistical analysis

32

8/18/2019 BRM_ch16


%nternet Sampling is 3ni0ue

• %nternet sur'eys allow researchers torapidly reach a large sample-

• Speed is both an ad'antage and a

disad'antage-• Sample si(e re0uirements can be met

o'ernight or almost instantaneously-

• Sur'ey should be "ept open longenough so all sample units canparticipate-

33

8/18/2019 BRM_ch16


%nternet Sampling

• Eaor disad'antage – Lac" of computer ownership and %nternet

access among certain segments of thepopulation-

• Jet %nternet samples may berepresentati'e of a target populations-

– )arget population @ 'isitors to a particular

/eb site-

• Bard to reach subects may participate-

34

8/18/2019 BRM_ch16


nd

35

Documents

BRM_ch16