note rm :sampling

8/7/2019 Note RM :Sampling

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SAMPLING

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INTRODUCTION It might be impossible to investigate everybody in a

population Thus, you need to select a sample of individuals

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INTRODUCTION Suppose a Professor A wants to study parents perception on

ETeMS via survey

It is impossible to distribute the questionnaire to every singleparent in Malaysia i.e. the population

,

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POPULATION SAMPLE

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SAMPLE & POPULATION Sample the group where the information is obtained

Population the group to which the results will be applied Thus, you have to identify the population 1st before the

sample

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SAMPLE & POPULATIONExample of population and sample

All teachers in Malaysia Sample- 200 teachers from each state

All math teachers in Malaysia

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amp e ma eac ers rom eac s a e All science teachers in Selangor

Sample 10 science teachers from each district

All students in the ED722 program Sample could be everybody since the population is small

Etc


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SAMPLING The process of selecting a number of individuals for a study

in such a way that the individuals represent the larger groupfrom which they were selected

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PURPOSE FOR SAMPLING

To gather data about the population in order to make an

inference that can be generalized to the population

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Regarding the sample

POPULATION (N)

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SAMPLE (n)

IS THE SAMPLE

REPRESENTATIVE?


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RANDOM & NON RANDOM SAMPLING

Random

Every member in the population has equal chance of beingselected

E.g. The dean of FP wants to investigate how all students (2000)feel about class time table

He selects 200 by picking out names from a box Non Random

Not all have a chance of being selected (also called purposive)

The dean decides to select 25 from 10 groups of students

Only semester 6 students

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RANDOM SAMPLING METHODS

Simple Random Sampling

Strati ie Ran omSampling

Cluster Sampling

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SIMPLE RANDOM SAMPLING Simple random sampling

The process of selecting a sample that allows individual inthe defined population to have an equal and independentchance of being selected for the sample

relatively homogeneous for the characteristic of interest.

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STEPS cont.

1

Identify and define the population

Determine the desired sample size.

3 List all members of the population.

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Assign all individuals on the list a consecutive number from zero to the requirednumber.

Each individual must have the same number of digits as each other individual.

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STEPS

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Select an arbitrary number in the table ofrandom numbers.

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For the selected number, look only at the number ofdigits assigned to each population member.

7 If the number corresponds to the number assigned to any of the individuals

in the population, then that individual is included in the sample.

8 Go to the next number in the column and repeat step #7 until the

desired number of individuals has been selected for the sample.

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SIMPLE RANDOM SAMPLING Advantages

Easy to conduct

Strategy requires minimum knowledge of the population to besampled

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Need names of all population members

May over- represent or under- estimate sample members

There is difficulty in reaching all selected in the sample


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STRATIFIED SAMPLING The process of selecting a sample that allows identified

subgroups (strata) in the defined population to berepresented in the same proportion that they exist in thepopulation

In other words ou used this when ou want certain rou s

to be represented

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STRATIFIED SAMPLING For example, you are interested in visual-spatial reasoning

and previous research suggests that men and women willperform differently on these types of task.

So, you divide your sample into male and female membersand randoml select e ual numbers within each sub rou or

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"stratum"). With this technique, you are guaranteed to have enough of

each subgroup for meaningful analysis.


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STRATIFIED SAMPLING

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STRATIFIED SAMPLINGAnother example

A researcher wants to investigate the use of technology inthe science classroom among teachers in Selangor

There are 1000 science teachers where 700 are females and

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To ensure that both genders are represented he will usestratified random sampling


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STEPS Identify and define the population.

1

Determine t e esire samp e size.2

Identify the variable and subgroups(strata) for which you want toguarantee appropriate, equalrepresentation.

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STEPS cont.

Classify all members of the

population as members of oneidentified subgroup.5

Randomly select, using a table ofrandom numbers) anappropriate number ofindividuals from each of thesubgroups, appropriate meaningan equal number of individuals

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STRATIFIED SAMPLING Back to the research on use of technology in the science

classroom

The researcher decides to take 35% from both gender

Thus, 245 female and 105 male science teachers will be

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STRATIFIED SAMPLING Advantages

More precise sampling

Sample represent the desired data

Disadvantages

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Difficulty in reaching all


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CLUSTER SAMPLING The process of randomly selecting intact groups, not

individuals, within the defined population sharing similarcharacteristics

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CLUSTER SAMPLING Cluster sampling is useful when it would be impossible or

impractical to identify every person in the sample.

Suppose a college does not print a student directory. It wouldbe most practical in this instance to sample students fromclasses. Rather than randomly sample 10% of students from

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eac c ass, w c wou e a cu t tas , ran om ysampling every student in 10% of the classes would be easier.

Sampling every student in a class is not a random procedure.However, by randomly selecting the classes, you have a

greater probability of capturing a representative sample ofthe population.


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CLUSTER SAMPLING This method is used when time and/or cost is a factor

E.gThe use technology in science classroom

The reseacher decides to select certain schools from eachdistrict

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us, a e sc ence eac ers n e c osen sc oo s w eselected

Thus, the teachers in the school constitued the cluster


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STEPS

Identify and define thepopulation

Determine the desiredsample size.

Identify and define alogical cluster.

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STEPS cont.

Determine the number of clusters needed by dividing thesample size by the estimated size of a cluster.

Randomly select the needed number of clustersby using a table of random numbers.

Include in your study all population members ineach selected cluster.

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CLUSTER SAMPLING Advantages

Efficient

Do not need to get all the names of the population

Reduces travel time

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Disadvantages Less likely to be representative


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RANDOM SAMPLING

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RANDOM SAMPLING

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NON RANDOM SAMPLING METHODS Systematic sampling

Convenience sampling

Purposive sampling

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SYSTEMATIC SAMPLING The process of selecting individuals within the defined

population from a list by taking every Kth name.

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SYSTEMATIC SAMPLING The researcher needs a sample of 200 from 2000.

So the tenth person on the list will be selected

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SYSTEMATIC SAMPLING

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STEPS

Identify anddefine thepopulation.

1 Determinethe desiredsample size.

2

Obtain a list

of thepopulation.3

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STEPS cont.

Determine what K isequal to by dividingthe size of thepopulation by thedesired sample size.

Start at some randomplace in the populationlist. Close you eyesand point your fingerto a name.

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STEPS cont.

Starting at that point,take every Kth nameon the list until thedesired sample size isreached.

If the end of the list isreached before thedesired sample isreached, go back tothe top of the list.

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SYSTEMATIC SAMPLING

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CONVENIENCE SAMPLING The process of including whoever happens to be available at

the time

Called accidental or haphazard sampling

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CONVENIENCE SAMPLING A researcher wants to investigate how teachers feel about

their COLA allowance

The reseacher waits at the punch machine and distribute thequestionnaire to the firts 30 teachers who came to punch in.

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PURPOSIVE SAMPLING The process whereby the researcher selects a sample based

on experience or knowledge of the group to be sampled

Called judgment sampling

Disadvantage

o en a or naccuracy n e researc er s cr er a an resu ng

sample selections

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PURPOSIVE SAMPLING Purposive sampling targets a particular group of people.

When the desired population for the study is rare or verydifficult to locate and recruit for a study, purposive samplingmay be the only option.

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,

processing speed of young adults who have suffered closedhead brain injuries in automobile accidents. This would be adifficult population to find.


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PURPOSIVE SAMPLING Your city has a well-established rehabilitation hospital and

you contact the director to ask permission to recruit from

this population. Purposive is different from convenience the researcher use

ud ment to select the sam le NOT who ever is available

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PURPOSIVE SAMPLING

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STEPS IN SAMPLING1. Define population (N) to be sampled

2. Determine sample size (n)3. Control for bias and error

. e ec samp e

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DEFINING THE POPULATION Identify the group of interest and its characteristics to which

the findings of the study will be generalized

called the target population (the ideal selection) oftentimes the accessible or available population must be

used (the realistic selection)

Examp e o target popu ation All primaryTamil school principals in Malaysia (350)

All parents in the state ofTerengganu (15,675)

Example or accessible population

PrimaryTamil school principals in the state of Selangor Parents in the district of KualaTerengganu

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DETERMINE THE SAMPLE SIZE The size of the sample influences both the representativeness

of the sample and the statistical analysis of the data

larger samples are more likely to detect a differencebetween different groups

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Rules of thumb for determining the


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Rules of thumb for determining the

sample size... The larger the population size, the smaller the percentage of

the population required to get a representative sample

For smaller samples (N 100), there is little point insampling. Survey the entire population.

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should be sampled. If the population size is around 1500, 20% should be

sampled.

Beyond a certain point (N = 5000), the population size isalmost irrelevant and a sample size of 400 may be adequate.

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SAMPLING BIAS Bias systematic error

Sampling bias consistent error arises from sample selection

Example: You want to investigate teenagers addiction to drugs

You select onl students in schools biased

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You should not forget teenagers who are not in school, dropouts

Biased when one group is over or under represented

Over example above

Under selecting from telephone, does not represent thosewho do not have land lines or choose not to list


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Control for sampling bias With bias your data may not be accurate

Be aware of the sources of sampling bias and identify how to

avoid it

Decide whether the bias is so severe that the results of the

In the final report, document awareness of bias, rationale forproceeding, and potential effects

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Select the sample A process by which the researcher attempts to ensure that

the sample is representative of the population from which it

is to be selected

requires identifying the sampling method that will be used

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APPROACHES TO QUALITATIVE


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APPROACHES TO QUALITATIVE

SAMPLING... Qualitative research is characterized by in-depth inquiry,

immersion in a setting, emphasis on context, concern with

participants perspectives, and description of a single setting,not generalization to many settings

Because samples need to be small and many potentialpart c pants are unw ng to un ergo t e eman s o

participation, most qualitative research samples arepurposive

Representativeness is secondary to the quality of the

participants ability to provide the desired information aboutself and setting

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TYPES Intensity sampling: selecting participants who permit study of

different levels of the research topic

Information rich cases E.g. In depth study of good students, poor students, average, below

average

Homogeneous samp ing: se ecting participants w o are very

similar in experience, perspective, or outlook E.g. Indian students only, single parents only

Criterion sampling: selecting all cases that meet some pre-

defined characteristic E.g.All teachers with B.Ed from UPM, all students with iPOD

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TYPES Snowball sampling: selecting a few individuals who can

identify other individuals who can identify still other

individuals who might be good participants for a study

E.g homeless people introducing other homeless people

Random purposive sampling: with a small sample, selectingby random means participants who were purposivelyselected and are too numerous to include all in the study

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Mini-Quiz

True or false

between convenience sampling andpurposive sampling

false


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True or false

researchers who use samples mustprovide detailed information aboutthe purposive research participants

and how they were chosentrue


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True or false

both the representativeness of thesample itself and the statisticalanalysis of study data

true


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True or false

research

false


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True or false

sampling biastrue


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True or false

convenience sampling andpurposive sampling is that, in thelatter, clear criteria guide selection of

the sample true


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True or false

representative of the populationfrom which it was selected

true


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True or false

,

researcher selects KKfalse


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True or false

the sample through a purely random,or chance, basis

true


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True or false

the researcher to describe in detailthe methods used to select a sample

false


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True or false

samples because the researcheruses clear criteria (e.g., experienceand prior knowledge) to identify

criteria for selecting the sampletrue


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True or false

strategies that produce sampleswhich are predominantly small andnonrandom

true


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True or false

precisely identical to that of thepopulation

false


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True or false

only feasible method of selecting asample because the population isvery large or spread out over a wide

geographic area true


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Fill in the blank

characteristics of is membersheterogeneous


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Fill in the blank

is completed in stages, involving theselection of clusters within clusters

multistage


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Fill in the blank

findings from a smaller group aregeneralized to a larger group

inference


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Fill in the blank

the sampledemographics


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Fill in the blank

characteristicshomogeneous


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Fill in the blank

are generalizablepopulation


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Fill in the blank

researcher finds an intact group ofsimilar characteristics (i.e.,population members)

cluster


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Fill in the blank

one study can be applied to otherpopulations or situations

generalizability


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Which type of sample

identified subgroups in the

stratified

popu a on are represen e n e

same proportion that they exist inthe population


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selecting a few individuals who can

snowball

en y o er n v ua s w o can

identify still other individuals whomight be good participants for astudy


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selecting participants who permit

intensity

s u y o eren eve s o e

research topic


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selects intact groups, not individuals

cluster

av ng s m ar c arac er s cs


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selecting by random means

random purposive

par c pan s w o are se ec e upon

defined criteria and not who are toonumerous to include all participantsin the study


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Which type of sampleselecting participants who are very

homogeneous

s m ar n exper ence, perspec ve,

or outlook


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Which type of sampleall individuals in the defined

random

popu a on ave an equa an

independent chance of beingselected for the sample


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Which type of samplea sampling process in which

systematic

n v ua s are se ec e rom a

list by taking every KKth name


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Which type of sampleselecting all cases that meet some

criterion

spec c c arac er s c

Thi d l h f d


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This module has focused on...

sampling a population

which describes the proceduresresearchers use to select individualsto participate in a study

Th t d l ill f


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The next module will focus on...

instruments

...the tools researchers use to gatherdata for a study

note rm :sampling

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