sampling process and data collection

8/18/2019 Sampling Process and Data Collection

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Sampling Process and Data Collection

Sampling

In an experimental study covering the entire population is not possiblebecause it is costly, time consuming, laborious and complication forhandling data. Hence it is preferable to use an appropriate sampling

technique.

(

i) Population statistical population!universe is the aggregate (totality) of

measurements on characters under investigation.

"or example

Population of doctors in a hospital, #omen of child bearing period ($%&') in a village, smoers in a city, cancer patients in a district etc.

sample(ii) Sample small part (*nite sub set) of statistical

Individuals chosen from the population for

studying its properties is called a sample.

+he number of units in a sample is no#n

as sample sie.


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+ypes of sampling techniques

The mechanism of sampling design to be used in a samplesurvey depends on the objective, scope of the inquiry, nature ofthe universe, size of the sample and available resources.

asically sampling procedures are of t!o types"

Non - random (Non-probability) sampling

#n non$random sampling selection of sample units are based onpersonal opinion and vie!s !here as in random samplingsample units are selected in accordance of some speci%edprobabilistic la!.

• Purposive & Subjective & 'udgment Sampling

• sno! ball sampling

• (uota Sampling


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Random sampling

•

Simple random sampling• Strati%ed random sampling

• Systematic sampling

•

Cluster sampling• )ulti$stage sampling


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-on&random sampling

The sampling techniques encountered under non$random samplingare"

Purposive sampling

#n purposive sampling, the sampling units are chosen !ithoutfollo!ing any scienti%c la! or theory so as to meet a de%nitepurpose. ecause of victim of bias due to element ofsubjectiveness viz. favoritism and nepotism of the investigator

this technique, in general, does not provide a representativesample of the population and is seldom use in practice. #n thismethod, the sampler*s choice regarding the items remainssupreme. +o!ever, if the investigator is !ell eperienced andthe -no!ledge of the population is available then purposive

sampling may yield valuable results.or eample $

/ sample of syringe selected by the seller for inspection0gallop pull of individuals belonging to a particular group etc.


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Sno#ball sampling

Sno#ball sampling " if the sampling frameand population is not !ell de%ned then non ofsampling starategy !or-s. #n such case !e%rst locate any sampling unit at any cost and

ta-e help of this sampling unit to %nd anotherunits and so on until !e get appropriate no ofsamples. /s the size of sample gets largerand larger each time li-e that of the size of

the sno!ball this method of sampling is-no!n as sno!ball sampling. This type ofsampling is done for cases li-e ta-ingresponses !here the respondent do not li-e

to reveal their identity.


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Quota Sampling

Quota Sampling

/ quota sampling is a special form of strati%edsampling. #n this method, the population is %rstdivided into several strata and the sample unitsusually -no!n as quota is selected from

di1erent strata according to the convenience ofthe sampler so as to meet the predeterminedsample size.

or eample $

Public opinion poll from n individuals consistingn2 technicians, n3 administrators, n4 laborers,etc. regarding in favor or against nuclear po!erplant.


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andom sampling!Probability sampling

andom sampling!Probability sampling

• The sampling techniques encountered under probabilityrandom sampling are"

• Simple random sampling

• De%nition $ / sampling scheme !herein each and every unit ofthe population has an equal chance of being included in the

sample is -no!n as simple random sampling.• (i) Simple random sampling #ithout replacement

(SS/0)

• S5S675 is a sampling scheme in !hich sampling units aredra!n successively one at a time follo!ed by no replacement

of the dra!n unit.• (ii) Simple random sampling #ith replacement

(SS/)

• S5S65 is a sampling technique in !hich sampling units aredra!n successively one at a time follo!ed by the replacement

of the dra!n unit.

) h d f d i l i i l


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)ethod of dra!ing samplein simplerandom Sampling

• (a) Lottery method

• Suppose, an investigator !ishes to put ontrial 38 patients out of 288 in a randommanner. 9nder lottery scheme the procedureis to select 288 slips, !hich are identical in allrespects :shape, size, color etc.;, and notethe 38 patient*s name on it :one name oneach slip;. The slips are then placed in a

container or bag, !ell shu


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Random number method

• Random number method

• This method is specially preferred !hen the size of the population is large. or this!e consider a random table prepared by =endal and Smith !hich consists of288,888 digits grouped into 3>,888 sets of ? digital random numbers. Thesenumbers are obtained by using a specially constructed machine, a re%ned version ofthe common roulette !heel used in gambling. @iven belo! is the sample of 3>random numbers of =endal and SmithAs random series"

• 342> 22B? 8B8 83? B4

• 8>>? ?44E ?442 B> 2E

• 2?B 48 E2>B 4B4 3>E• 4B ?>? 424> B3E3 2??

• B42 4EBE >B8? E283 2243

• The popularity of the random number series lies in the fact that they can be read onany page, at any point and in any direction :ro! !ise, column !ise or diagonally;.Suppose !e !ant to obtain a random sample of n F 38 patients out of G F 288

patients. irst assign serial numbers to all 288 :2 to 288; patients at random. Get apage of a random number is turned out and starting from any point and in anydirection horizontally or vertically, 38 three digital %gures are recorded by ignoringthe %gures greater than 288 or for any number greater than 288, divide by 288 andta-e the remainder as sample number i,e if the selected number is 3?>, the randomsample number is ?> and so on. The 38 patients corresponding to those 38 selectednumbers constitute the desired random sample.


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Stratifed random sampling• 1e*nition

• / strati%ed random sample is a random sample obtainedby categorizing the population elements into non$overlapping groups, called strata, and then selecting asimple random sample !ithin each stratum.

• This technique is specially recommended !hen thepopulation under investigation is heterogeneous. 9nderthis method the heterogeneous population under study is%rst divided into relatively homogeneous groups calledstrata in accordance of certain characteristic such as

blood pressure :+igh, medium or lo!;, patient type:cancer, tuberculosis or diabetics ;, economic status: rich, middle class or poor ; etc. and a sample is dra!nfrom each stratum at random in proportion to its size. Theaggregate of selected units from di1erent strata represent

the required sample units.


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Systematic sampling

• This method is most !idely used in %eld studies !hen the

population is large, scattered, heterogeneous and acomplete list of population is available.

• Suppose !e are interested to select a sample of size n F28 H of 488 F 48 vaccine bottles out of 488 vaccine

bottles in a systematic sampling technique. or that !ehave 28 H of 488 F 48 units so that sampling intervalbecomes 6here G F population size

• Get the !hole vaccine bottles are arrangedsystematically, assign numbers serially from 2 to 488,

divide into 48 groups of 28 vaccine bottles each. Get arandom number is chosen by simple random samplemethod. Suppose the chosen number is ? then the sample!ill consist of 48 vaccine bottles possessing numbers ?, 2?,3?, 4?, ??, >?, E? and so on from 2st., 3nd., 4rd group

respectively !hich provides 48 sample vaccine bottles.


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Cluster sampling

1e*nition

• / cluster sampling is a S5S in !hich samplingunit is a collection or cluster of elements.

• #n a certain district having 388 schools if 28schools are chosen under S5S technique andenumerate all the units !ithin the selectedschools, then, the schools so selected are-no!n as clusters and the technique is -no!n

as cluster sampling.• / cluster sampling !herein geographical

areas are used as clusters is -no!n as areasampling.


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ultistage sampling

1e*nition

• #n a multi stage sampling mechanism in !hich sampling

procedures are carried on in several stages using randomsampling techniques. This technique is recommendable for largecountry surveys.

• echanism

• Suppose !e are handling a stage !ise survey of status of eye

defective patients"• Iet us consider the B> districts of Gepal.

• 2st stage" Select ?8 districts of Gepal S5S technique.

• 3nd stage" Select 38 JDC& Gagar Pali-a from the selected ?8districts by S5S technique.

• 4rd stage " Select 8 households out of 38 JDC& Gagar Pali-a byS5S technique.

• ?th stage " inally enumerate all eye defective patients in theselected households. This is a four stage sampling.

•


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Types of errors in Data Collection

• Sampling Lrror

• Gon sampling Lrror


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Gon sampling Lrror

• #naccurate reporting by therespondent

• /ctual lying by respondents

• Poor Sampling Design

• )isintrepretation of question due toambiguous !ording

• +esitant 5espondent

• Coding and Lditing Lrrors


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Sampling Lrror

• Lrror incurred by selection of sample thatare not representative of the population

Sources of Sampling Lrrors

$aulty selection of sample$Substitution

$aulty Demarcation of Sampling 9nits

$Constant error due to improper choice ofthe statist tics for Lstimating populationparameter


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/dvantages of Sampling overCensus

• Iess time

• 5educed cost of survey

• @reater accuracy of results

• @reater scope

• #f the population is too large or

• #f the test is destructive or

• #f the population is hypothetical then onlypossible method of data collection issampling


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• Intervie#

• #t is a technique of primary data collection. #t is an oral method in !hich oneperson as-s another person questions designed to obtain ans!er pertinent tothe 5esearch problem. #t is most commonly used direct method in the studyof human behavior. The intervie! is a face to face interpersonal role situationin !hich one person as-s another person being intervie!ed. The respondent

question designed to obtain ans!er pertinent to the purpose of the 5esearchproblem. The intervie! may be regarded as a systematic method by !hichone person enters more or less imaginatively !ith the inner life of another!ho is

• generally a comparative stranger to him. The purpose of intervie! is to %ndout !hat*s in or on someone else*s mind.

•

ccording to !erlinger 2+he intervie# is a face to face interpersonalrole situation in #hich one person, the intervie#er, ass a personbeing intervie#ed, the respondent, questions designed to obtainans#ers pertinent to the purpose of the esearch problem.3

•


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• +ypes of intervie#

• M Structured or standardi"ed intervie#

• #t this intervie!, the questions, their sequence and their !ordings are %ed. Thesame question is as-ed to everybody in the same manner. in general, theintervie!er doesn*t have Neibility to change the question, their format or order.Sometimes an intervie!er may be allo!ed some liberty in as-ing question but

relatively little. That liberty is speci%ed in advance. Standardized intervie!s areintervie! schedule that have been carefully prepared to obtained informationpertinent to the 5esearch problem.

• M $nstructured or unstandardi"ed intervie#

• #n the intervie!, the sequence and !ording of question are not %ed. Theintervie!er doesn*t follo! pre$planed list of the question. The intervie!er !ritesthe response

• of the intervie! during the intervie! or after intervie! is completed. #t is moreNeible and open. 7rdinary no schedule is used. They don*t mean thatunstructured intervie! is casual. The intervie!er !ill have a tentative list ofquestion to be covered during the intervie!.

•


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• Questionnaire

• / questionnaire is a format list of questions designed to gather respondent fromrespondent on a given topic. #t is a formalized schedule for data collection. #t isused !hen fractional information is designed. 6hen opinion rather than facts aredesired, opinionnaire or attitude scale is used. (uestionnaire is a term used foralmost any -ind of instrument that has questions or items to !hich individuals

respond. / questionnaire is a list of questions to number of persons for them toans!er. #t secures standardized results that can be tabulated created statistically.

• +ype of questionnaire

• M %pen or open-ended &uestion'

• M Close or close-ended &uestion'

• restricted questionnaire

• ypes o close- &uestionnaire

• M *ichotomous &uestionnaire'

• #t o1ers only t!o alternatives0 positive and negative. The response !ould be yes orno0 agree or disagree0 and so on. #t is also -no!n as t!o !ay questionnaire.

• M ulti chotomous &uestionnaire'

• #t is also called multiple choice questionnaires.


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• 5uestionnaire 1esign

• / questionnaire is designed to collect right type of primary data of right quality. The design of questionnaire ismore than art from than a scienti%c underta-ing. (uestionnaire design comes from the eperience of the5esearchers !ho specialized in this area. The only !ay to develop this s-ill is to !rite a questionnaire, use it in aseries of intervie!s, analyze its !ea-ness and revise it. There are no series of step, principle or guidelines !ithguarantee on e1ective and eOcient questionnaire. /n e1ective questionnaire is one that

• M Dra!s out accurate information

• M Cab be completed easily by intervie!ers

•

M lo! !ell• M Ieaves the respondent feel satis%ed for their !orth !hile participation in the 5esearch

• There are series of seven steps in questionnaire design as

• started belo!"

• #t is versatile. /lmost every problem of social 5esearch can be approached from the questionnaire stand point.Lvery social problem involves people. Therefore, ideas relative to the problem and solution can be obtained byas-ing these people about the problem

• )any people can be studied only by questioning. =no!ledge, opinions, motivation, and intentions are usuallynot open to observation. Similarly, it is not feasible to observe personnel activities such as burning teeth. So, it is

feasible.• (uestioning is usually faster and eOcient than observation. Some events that ta-e place over a time period

!ould require lengthy observation, but a question on this behavior can be ans!ered in a fe! seconds.

• #t is cheaper then observing. The 5esearcher has not to stand time to observe the behavior of the respondent./ decrease in time usually lead to decrease in cost.

• The person administrating the questionnaire has an opportunity to establish report, eplain the purpose of thestudy and eplain the meaning of items that may not be clear.


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• 1isadvantages of questionnaire

• M 5espondents !ould be un!illing to provide information. (uestions aboutincome or very personnel subject frequently meet refusal by respondent. Thesho! little or no interest to the question.

• M Despite a !illingness to cooperate, many people are unable to give accurateinformation of questions.

• M There can be inNuence on questioning process. 7ften respondent attempt togive ans!ers that they thin- !ill please the 5esearchers.

• M illing out lengthy questions ta-e a great deal of time and money.

• 0ther disadvantages

• M #t cannot be used in illiterate person.

• M #ncomplete responses are possible

• M There is a possibility of !rong ans!er• M 9seless in depth problem

• M 9niform questions are not applicable to other people of di1erent ethnicity,culture etc.

•


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• %bservation

• #t is a method of gathering primary data physically ormechanically recording events or aspect of the phenomenonunder investigation. #t involves recording of the respondents*

• behavior. #t is the process of recognizing noting people, objects

and occurrence of events rather than as-ing for information. #t cansupplement the information collected through

• questionnaire and intervie!. 7bservation is the process ofrecognizing and recording

• behavior of people, objects and events. 7bservation is systematic

and deliberate study through eye, of spontaneous occurrence atthe time they occur. Lample" instead of as-ing consumer !hatbrand they buy or !hat television program they !atch, the5esearcher arrange to observe !hat product are brought and !hatprogram they !atch.


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• 6haracteristics of good observations

• M 7bservation should be carefully planned, systematic and perceptive.7bservers should -no! !hat they are loo-ing for and !hat is irrelevant in asituation.

• M #t should focus on !holeness of !hat is observed.

• 7bservers should not only be alert to signi%cant details, they should also-no! that the !hole is often greater than the sum of its parts.

• M #t should be objective and bias$free. 7bservers should strive to eliminatetheir inNuence !hat see and report.

• M #t should separate the facts from the interpretation of facts. 7bserversobserve the facts and ma-e their interpretation at a later time.

• M #t should be chec-ed and veri%ed !henever possible by repetition or by

comparison !ith those of other competent observers.• M #t should be carefully and epertly recorded. 7bservers use appropriate

instruments to systematize, qualify and preserve the result of theirobservations.

• M 7bservations are collected in such a !ay that they are valid and reliable.


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Determionation of Sample Size

• $Sample Size for LstimatingPopulation )ean on the base ofsample mean

• $Sample size for estimating Polulationproportion on the basis of Sampleproportion


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Sample size to determinepopulation mean

• Gecessary information

• Q value :Lither given or to bedetermined by yourself;

• Sample variance or Populationvariance

• )argin of Lrror:D or L;

• Then use formular


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Sample size to determine Populationvariance

• Gecessary information

• Q value :Lither given or to bedetermined by yourself;

• Sample variance or Populationvariance for propoertion:p and q;

• )argin of Lrror:D or L;

• Then use formulae


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Sample Sie

+oo 7ig8

•equires

too much

resources

+oo

small8•/on9tdo the

:ob

; l S l


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•What sample size is needed to be 90%

confident of being correct within ± 5?

A pilot study suggested that thestandard deviation is 5

;xample8 SampleSie for


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•What sample size is needed to be within

± 5 with 90% confidence? !ut of a

population of "#000# we randomlyselected "00 of which $0 were defective

;xample8 SampleSie for Proportion

ound

( ) ( ) ( )

2 2

2 2

1 1.645 0.3 0.7

Error 0.05

227.3 228

Z p pn−

= =

= ≅


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• / manufacturing concern !ants toestimate the average amount ofpurchase of its product in a month by

customers. The standard deviation ofpurchases is 5s.288.ind the samplesize if the maimum error is not to

eceed 5s.4 !ith a probability of8.>:zRF2.E;


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• The bajaj manufacturing sells a tubelight that has mean life of ?4>8 hours!ith a standard deviation of >8

hours./ ne! manufacturing processis being tested and there is interestin -no!ing the mean life of of the

ne! tube lights. +o! large thesample size is required so that >is a >Hcon%dence interval for U

Vou ma assume that the change in

sampling process and data collection

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