Determining Sample Size

Session 14: Determining Sample Size

Total Session Time: 120 minutes + 60 minutes assignment

Prerequisites

Learning Tasks

By the end of this session students are expected to be able to:

Resources Needed

 Flip charts, marker pens, and masking tape

 Black/white board and chalk/whiteboard markers

 Computer and LCD Projector

SESSION OVERVIEW

Activity/

Step Time Content

Method

1 05 minutes Presentation Introduction, Learning Tasks

Presentation

2 15 minutes Issues to Consider when Deciding on Sample Size

Buzzing

50 minutes Presentation

3 Calculation of the Sample Size (Desirable)

Brainstorming

4 35 minutes Presentation Determination of Sample Size(s) Most Appropriate for

the Research Design (Feasible)

5 Key Points

05 minutes Presentation

6 05 minutes Presentation Evaluation

7 05 minutes Presentation Assignment

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SESSION CONTENTS

STEP1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing

STEP 2: Issues to Consider when Deciding on Sample Size (15 minutes)

Activity: Buzzing (5 minutes)

ASK students to pair up and buzz on the following question for 2 minutes:

 What issues should be considered when deciding on sample size?

ALLOW few pairs to respond and let other pairs to add on points not mentioned

WRITE their response on the flip chart/board

CLARIFY and SUMMARIZE by using the content below

depends on WHAT you try to find out, WHY you try to find out, and HOW you are going

to use the data

 the sample size depends on the type of problem investigated, required precision and to a

certain extent, the resource available

 If your research objective is more complex e.g., attitudes of males and females towards

family planning, and has policy implications for a larger area, your sample will be bigger.

 Very few studies use the whole population (Census)

 Most studies use a subset of a population (Sample)

 How big should a sample be?

 Too large or too little sample is unethical:

 Too large: waste resources

 Too little: Inconclusive results

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STEP 3: Calculation of the Sample Size (Desirable) (50 minutes)

Activity: Brainstorming (10 minutes)

Ask students to brainstorm on the following question:

ALLOW few students to respond?

WRITE their responses on the flip chart/ board

CLARIFY and SUMMARISE by using the content below

study:

o The desirable sample size depends on the expected variation in the data (of the most

important variables): the more varied the data are, the larger the sample size we would

need to attain the desired level of accuracy.

o The desirable sample size also depends on the number of cells we will have in the

cross-tabulations which we need to analyze the results.

 A rough guideline is to have at least 5 to 10 study units per cell

o How big can be calculated. Still, the desirable sample size cannot always be achieved

for lack of resources such as time, manpower and money.

 This constraint applies to quantitative as well as qualitative studies

 Therefore, the eventual sample size is usually a compromise between what is

desirable and what is feasible

o Sample size calculations

` The following steps should be taken:

 Estimate how big the proportion might be (say 80%)

 Choose the margin of error you will allow in the estimate of the proportion (say +

10%). This means that, if in the survey indeed 80% of the children are found to be

vaccinated, this proportion will probably be between 70 and 90% in the whole

study population from which the sample was drawn.

 Choose the precision with which you want to be confident that the vaccination

coverage in the whole population is indeed between 70 and 90%. You can never be

100% sure. Do you want to be 95% sure? Or 99%?

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o Formulae

d2

STEP 4: Determination of Sample Size(s) Most Appropriate for the

Research Design (Feasible) (35minutes)

 The feasible sample size determination

o Is not calculated, depends on available resources (human, time, money, transport)

 Availability of finance: in practice, size of the sample depends upon the amount of

money available for the study purposes. This factor should be kept in view while

determining the size of sample for large samples result in increasing the cost of

sampling estimates

o Depends on expected variation in the data: the more varied the data are, the larger the

sample size one would need to attain the same level of accuracy

o Depends on the number of cells one will have in the cross-tabulations required to

analyze the result

 A rough guideline is to have at least 20-30 study units per cell

o Eventual sample size – is a compromise between desirable and feasible

o Nature of study: If items are to be intensively and continuously studied, the sample

should be small. For a general survey the size of the sample should be large, but a

small sample is considered appropriate in technical surveys.

o Type of sampling: Sampling technique plays an important part in determining the size

of the sample. A small random sample is appropriate to be much superior to a larger

but badly selected

.

STEP 5: Key Points (5 minutes)

 There are no fixed rules for sample size in qualitative research.

 Eventual sample size is a compromise between desirable and feasible

STEP 6: Evaluation (5 minutes)

 What are issues to consider when deciding on sample size?

 What is the sample size(s) most appropriate for the research design (feasible)?

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STEP 7: Assignment (5 minutes)

Activity: Take Home Assignment (5 minutes)

DIVIDE students in groups or individuals

ASK the students to work on the following Assignment

Determine the sample size of the research proposal being developed

ALLOCATE time for students to do the assignments and submit

REFER students to recommended reference

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References

References

Hardon A, Boonmongkon P, and Streefland P. et al (2001). Applied Health research,

Anthropology of health and health care, (3rd Ed) Amsterdam, The Netherlands: Het

Spinhuis Publishers

Beaglehole R, Bonita R and Kjellstrom (1993) Basic epidemiology: Geneva, Switzerland:

World Health Organization,

Kothari C.R (1985). Research Methodology – Methods and techniques, (2nd ed); New Delhi,

India; Wiley Eastern Limited

Stewart A (2001). Basic Statistics and epidemiology, A practical guide,; London, United

Kingdom: Radcliffe Medical Press,

Varkevisser, C. M, Pathmanathan, I and Brownlee, A (1991). Designing and Conducting

Health Systems Research Projects, Vol. 2 Part I: Ottawa, Canada: IDRC

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