INTRODUCTION TO BASIC EPIDEMIOLOGY AND PRINCIPLES OF STATISTICS FOR TROPICAL DISEASE CONTROL

WHO/CDS/CPE/SMT/2000.2 Rev.1
Part II

Introduction to basic epidemiology
and principles of statistics for tropical
diseases control

Tutor's Guide

Communicable Diseases Cluster
Department of Control, Prevention and Eradication
Social Mobilization and Training Unit
Updated July 2002

Trial Edition


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Table of contents

Table of contents
Foreword ....................................................................................................................................3
Introduction................................................................................................................................5
Learning Units
1. Introduction to epidemiology...............................................................................................19
2. Rates, ratios, and proportions...............................................................................................21
3. Data presentation: tables, graphs and charts ........................................................................25
4. Measures of central tendency...............................................................................................35
5. Measures of variability and test of association; normal distribution ...................................39
6. Principles of surveillance.....................................................................................................43
7. Health facility-based epidemiological studies .....................................................................47
8. Surveys and data management .............................................................................................52
9. Assessing the accuracy of a test or surveillance system ......................................................52
Annex 1....................................................................................................................................52
Annex 2....................................................................................................................................52
Annex 3....................................................................................................................................52
Annex 4....................................................................................................................................52

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Introduction to basic epidemiology & principles of simple statistics for tropical disease control – Tutor’s guide

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Foreword

Foreword

This module uses a problem-solving approach to facilitate the learning of
some basic epidemiological concepts and practices and simple statistics. It is
designed for health workers responsible for tropical diseases control and is
considered to be fundamental to the learning of the epidemiological approach
to malaria control and for a situation analysis.
The module is designed to stimulate active learning. It is divided into two
parts: Learner's Guide (Part I) and Tutor's Guide (Part II). The Learner's
Guide contains basic information and exercises. The Tutor's Guide contains
suggestions for using this training module and provides suggested answers to
the exercises.
The module can be used in different ways. It is primarily created for group
work as one element of a comprehensive training course on basic malariology
and planning malaria control. The module can also be used separately for
epidemiology or in-service training at appropriate levels of the health services.
It can be a useful component of any programme for training in tropical disease
control at the district and national levels. The Learner's Guide can be used for
individual work. The Tutor's Guide is intended to supplement the tutor's own
knowledge and experience and to guide the facilitators, or, in the case of
individual study, to be used as an answer book. The training using this module
is designed to be accomplished in 26 hours including 1 hour each for the pre
and post tests (see proposed Timetable in the Introduction to this manual).
The module has been developed by Dr N. Binkin whilst on assignment to the
International Course for Primary Health Care Managers and the Laboratory of

Epidemiology and Biostatistics at the Istituto Superiore di Sanità in Rome.
Dr Binkin was at the time a WHO staff member of the Division of
Strengthening of Health Services, HQ, Geneva. Thanks go to Dr P.F. Beales,
the late Dr R.L Kouznetsov and Dr F.A. Rio for reviewing the document and
reorganizing it into a two-volume training module. The technical editing of the
module was undertaken by Dr F.A. Rio and pursued by Dr M. Aregawi and
Dr M.C. Thuriaux, all at WHO Headquarters. Participants and facilitators in
English-, French- and Portuguese-language courses in Nazareth (Ethiopia),
Cotonou and Maputo respectively have contributed greatly to the practical
development of the module, which was supported by a financial contribution
from the World Bank.

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Introduction to basic epidemiology & principles of simple statistics for tropical disease control – Tutor’s guide

4


Introduction

Introduction

This Tutor's Guide is designed primarily to help those responsible for training
health workers and tropical diseases control personnel in basic epidemiology
and simple statistics. Some parts of it should be useful even to the most
experienced teachers. The style of writing has been kept simple, to avoid
misunderstandings and to facilitate translation into local languages.
This introduction will help you understand the role of tutor and facilitator in

this training system and explain why the Learner's Guide is designed the way
it is. It is essential that you read the whole of the Learner's Guide (Part I of the
training module) before planning your training programme, rather than reading
only the Unit that relates to your next teaching session.

For whom is this training module intended?
The module is intended for health workers who are responsible for planning,
managing, implementing, evaluating or teaching the control of tropical
diseases, especially (but not only) malaria. It can be used alone for a special
course or as one element of a more comprehensive course on disease control. It
is not designed for the training of epidemiologists, for which several
publications are available (see list at the end of the Learner’s Guide).

Educational level of learners
The appropriate educational or entry level of learners will depend upon a
number of factors. Experience in many parts of the world has shown that
health workers from a wide range of educational backgrounds can be accepted
for training in this subject. However, if the entry level is relatively low, the
period of training may need to be extended. On average, it should be possible
to teach the contents of this module in four days. Apart from educational
requirements, it is equally important that learners:
· be able to read, understand and write simple English (or the language into
which the module is translated)
· be able to follow a set of written instructions systematically
· have good hearing and eyesight
· be sympathetic to the health problems of the community.

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Introduction to basic epidemiology & principles of simple statistics for tropical disease control – Tutor’s guide

This list is not necessarily complete: another requirement may for instance be
willingness to work for long periods in rural areas far from home. Often, it will
be impossible to interview candidates directly. It then becomes particularly
important, when writing to those who will select learners for the course, to
indicate the most suitable type of person.

How is the training designed and what are its contents?
The training module is intended to facilitate the teaching of basic
epidemiological tasks and the appropriate use of some simple statistics to
health workers and disease control personnel. The general objectives of the
course are:
1. To define and describe the role of health statistics in the implementation
of an epidemiological approach to tropical diseases control
2. To define and describe major types of descriptive and analytic studies,
their purpose, and their primary users
3. To define the usefulness and limitations of each type of epidemiological
study in drawing conclusions about disease problems
4. To describe the steps in planning, setting up and evaluating different types
of epidemiological studies
5. To examine, analyse and interpret data of epidemiological studies and
surveys
6. To acquire and apply practical skills in the use of various techniques and
tools for the calculation and interpretation of numerical information.
The training deals with basic epidemiology and simple statistics in a practical
sequence. For example, learners are taught rates, ratios, and proportions before
surveillance and surveys. In other words, the learners acquire, step by step, the
knowledge and skills they need for the tasks involved in basic epidemiology
and simple statistics. This type of training may be referred to as performancebased or competency-based. When carried out properly, it is highly effective.

It is also very economical: training is kept as short as possible, yet participants
learn all they need to perform the required tasks competently. This saves time,
money and resources.

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Introduction

A list of specific training objectives appears at the beginning of each Learning
Unit in the Learner's Guide. They summarize the knowledge, skills and
attitudes that each learner must have acquired by the end of that Unit. Before
proceeding to the next Learning Unit, the Tutor and Facilitators must satisfy
themselves that each learner has achieved the stated objectives (methods of
evaluating progress are described later). It is advisable to have each of the
participants take turns at reading these objectives aloud at the end of each
unit and to encourage those who consider they have not mastered the
objectives to revise the material in depth.
While it is more convenient to have all the learners working together or in
small groups on each Learning Unit, this programme allows for slower
learners to work through each Unit at their own pace. However, careful
planning is essential if the brighter participants are not to become bored.

Who runs the course?
The Tutor and Facilitators are responsible for organizing and running the
course. The Learner's Guide and Tutor's Guide will help you, but the final
results will depend upon the efforts of the Tutor and Facilitators. For some,
this may be the first time they organize and run such a course; others may be
experienced in these courses. In either case, it is important to use the Learner's
Guide and the Tutor's Guide together whilst proceeding through the Learning

Units.

Who helps in the course?
The Tutor’s job will be easier, and teaching more effective, if one or more
persons help. These assistants, who must have working experience in the
subject, are called Facilitators. Learners can thus be divided into small groups
of perhaps 5, allocating one facilitator to each group. The greater interaction
this allows between the learners and the facilitators results in improved
learning and understanding.
The Tutor will be responsible for designing the timetable, explaining the
learning tasks to the learners and facilitators, and giving learners and
facilitators whatever help they need. Facilitators need not necessarily be
trained as teachers: their task is to explain or demonstrate a particular activity
and to watch the learners perform it. They must also be able to admit to
learners when there is something that they do not know and be prepared to
refer the question or problem to the Tutor. Both Tutor and Facilitators must
keep in mind the fact that no one person can be expected to know everything
about a particular subject. There is no shame in saying " I do not know, but I
will find out for you".
Many problems can be avoided by giving facilitators plenty of time to read the
Learner's Guide and to discuss with the Tutor any part of it that may need
clarification. It would be a good idea for the Tutor and Facilitators to go

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Introduction to basic epidemiology & principles of simple statistics for tropical disease control – Tutor’s guide

through the module together; testing their knowledge through appropriate
questions.

There are many approaches to epidemiology and statistics. This training
module describes and attempts to standardize well-tried methods already
used in many parts of the world.

Why provide a Learner's guide?
Providing learners with a full set of notes ensures that:
· All learners have exactly the same set of notes, and thus avoid unnecessary
note-taking during lessons;
· Tutor and facilitators can refer to any part of the Learner's guide knowing
that all learners can find the right page quickly;
· Learners can spend more time reading the notes, and therefore have a
greater chance of understanding them, because there is no need to write up
notes taken during class;
· There is no chance of learners making errors in note-taking;
· After the course, each learner can take home a copy of the Learner's guide
and the Tutor's guide as a helpful reference in his or her daily work and
perhaps also to use in teaching others.

How is the course run?
Formal classroom presentations of information in the guise of lectures will
usually be kept to a minimum and each session will be as short as possible.
The information given in such sessions is already contained in the Learner’s
Guide, so there will be little need to take notes. A presentation will usually
include examples of epidemiological principles and/or statistical calculations.
Small group work will include working through exercises and examples. A
moderator chosen by the members of each group will lead discussions on
particular subjects. The sessions provide good opportunities for learners to
give their opinions, develop their ideas and learn from one another.

Learners who are actively involved learn more, and better, than those
who simply sit and listen to someone talking for long periods of time.

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Introduction

Use of the Tutor's Guide and Learner's Guide
The Tutor's Guide and the Learner's Guide may be used together for basic
group training and for in-service training. The Learner's Guide may be used
alone for refresher training, or by individuals for reference. Learners will
follow the group training activities using the Learner's Guide plus whatever
other materials you provide them with.

Training facilities
Basic facilities and equipment must be organized before training can begin. In
some countries these are readily available but in others it may be necessary to
improvise or modify existing equipment. Remember that there may be long
intervals between ordering supplies and getting them delivered; training should
not be delayed unnecessarily because equipment is imperfect or scanty.
Ideally, one large room must be available for training. The room can be used
for group discussions, presentations, and for the overhead and slide projectors
(it is an advantage if smaller rooms are also available for small-group work,
but this is not essential, see Arrangement of the meeting room hereafter).
Chairs and small tables or desks will be needed. Whatever the conditions,
learners must be as comfortable as is possible in the circumstances:
surprisingly much may be achieved even with limited facilities.

Arrangement of the meeting room

The number of working groups must be decided upon in advance. Groups of 6
to 8 are best. This will depend upon the number of learners and number of
facilitators available. The room must be arranged in such a way that
participants sit in groups, in more or less a semi-circle as in the diagram.
Everyone must have a clear view of the blackboard and projector screen.
The composition of each group can be changed occasionally or left the same
throughout the course. For the pre- and post-test evaluations, however,
participants must be seated apart from one another under examination
conditions. However, the group activities can all take place in the same room
and time is saved by not having to change places.

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Introduction to basic epidemiology & principles of simple statistics for tropical disease control – Tutor’s guide

Teaching equipment
For teaching sessions and group discussions, the following items should
ideally be available:
· Overhead projector.
· Screen for slide projection (a white sheet is an adequate substitute, but the
white-board is unsuitable because it will reflect projected light).
· Flipcharts—one for each small group of learners. Supplies of "butcher's
paper" or "newsprint" are usually cheap and readily available.
· Large blackboard or white-board.
· Chalks for blackboard or marker pens for white-board, in a selection of
colours.
· Acetate sheets for overhead projector.
· Coloured marker pens for acetate sheets (including some permanent
markers for diagrams you may wish to keep).

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Introduction

Learners' equipment
Each learner should receive the equipment listed below. Where supplies have
to be ordered, this must be done well in advance of the course, since many
items are difficult to obtain at short notice.
· Learner's Guide.
· Notebook. This must be used only for occasional notes or instructions (there
is normally little need for notes to be taken during training sessions)
· Arithmetic (squared) paper
· Semi-log paper
· Ballpoint pen.
· Set of pencils (medium-hard graphite, plus red, blue, brown and black) for
drawing charts and graphs during practical sessions.
· Pencil sharpener.
· Eraser.
· Ruler.
· Simple hand held calculator.

Syllabus and timetable
The syllabus
The Table of contents of the Learner's Guide represents the syllabus – the list
of subjects to be covered – for the training course. Planning the course is made
easier by the division of each Learning Unit into a number of subunits or main
topics. It is necessary for the Tutor to go through each of the Learning Units in
turn; and to decide for each subunit the amount of time needed and the most

suitable kind of training activity. For example, Learning Unit 1 – Introduction
to epidemiology – has 6 learning objectives and is divided into 4 main
sections. This unit can be dealt with through presentations and through
questions and answers. On the other hand, Learning Unit 2 – Rates, ratios and
proportions – has 5 learning objectives and 5 main sections. This unit needs to
be dealt with through presentation, numerous examples, individual and group
exercises involving the Facilitators and the Tutor. The following is a list of the
various learning activities that can be used:
· Group discussion
Once participants get used to group discussions, the two-way exchange of
information between them and the Facilitators makes this a very effective
learning activity. People share their knowledge and experiences with the rest of
the group and stimulate each other's thoughts on the subject in hand.
· Practical exercises
Practical exercises may be undertaken individually or in groups in the
classroom. Their purpose is to give learners the opportunity to practise the
procedures involved. The more practice learners have, the more competence
they will acquire.
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Introduction to basic epidemiology & principles of simple statistics for tropical disease control – Tutor’s guide

· Demonstrations, examples
These are designed to reinforce the learning process. Clear examples help to
clarify concepts and concretize principles of epidemiology and statistical
calculations and methods.

Timetable
Once the amount of time that needs to be spent on each subunit has been

calculated, the various learning activities must be fitted into the framework of
the training programme. The duration of the programme may be something
over which there is little control: shortage of funds may for instance limit the
programme to 3 days, even though it should ideally be spread over 4 days. The
Tutor and Facilitators will then need to spend time reorganizing the timetable
so that all the learning activities can be fitted into the time available.
The timetable must allow time for evaluation both during and after the course,
and for "hidden" activities, such as getting settled into group work, delays in
transportation to training facility and so on. A suggested timetable for a 4 day
training course is shown in the Figure at the end of this section, but is provided
only as a guide. It is based on a 7-hour working day—four hours in the
morning and three in the afternoon; this may not always be suitable and may
have to be adapted. Further discussion on some topics. These activities can be
fitted into the "free" break periods and a discussion session on the afternoon of
the last day can also be used in a flexible manner

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Introduction

Evaluation
Judging whether or not the course was a good one involves 2 main elements:
1. How well did the group learn?
This may be determined by evaluating the learners' performance as they work
through the Learning Units and again at the end of the training, by evaluating
the level of skill, competence, and knowledge that learners have achieved in
this subject. This may be done by the use of pre-and post-tests; examples of
questions to that purpose are to be found in Annex 1. More details on
evaluation are given later in this Guide. A further evaluation of how well the

learners have retained their knowledge, skills and competence may be required
10-12 months after the end of the course.
It is essential, both for the learners and for the Tutor and facilitators, to assess
progress made by the learners in gaining skills and competence in the subject
matter. This can be accomplished by means of pre- and post-tests. To be valid,
answers to these tests must be the learner's own work. Annex 2 provides
suggestions for pre- and post-test questions together with the answers. The
post-test must be administered only after all the learning units have been
completed. Since the answers to the evaluation questions, and to the exercises,
are included in the Tutor's Guide, it is essential that learners do not have
access to this Guide until after the training activity has been completed.
The results of the pre-test can be used in 2 ways. The Tutor may use it to
ascertain the general level of knowledge on the subject among the group, and
to obtain an indication of weak areas that need emphasis and areas of general
knowledge that can handled more lightly. The results may also be used to
identify individuals who may be used as Facilitators for certain subject areas.
The other major use of the pre-test is as an individual base-line comparator to
measure gains in knowledge, skills and competence at the end of the training
as revealed by the post-test.
To be valid, both tests must be given under the same conditions and the same
length of time; the questions in the post-test must be of the same difficulty as
the questions in the pre-test. The only sure way of ensuring that the questions
in the post-test are of equal difficulty to those in the pre-test is to give the same
questions in a different order (with the answers also in a different order in the
case of multiple choice questions). It is thus essential that the pre-test papers
be collected up and retained (not handed back to the participants). In any
event, it is not necessary for the participant to know the results of the pre-test
until the end of the training, when these results are used to determine progress.
The Tutor is encouraged to develop a bank of questions to be used for pre- and
post-testing in subsequent training sessions. The answers to sample pre- and

post-test questions are provided separately in this Tutor's Guide to allow easy
reproduction of question papers. The answers are scored equally because each
question is considered, in this instance, to be of equal value.

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Introduction to basic epidemiology & principles of simple statistics for tropical disease control – Tutor’s guide

2. How did the learners view the training?
Feedback helps assess how well the training is being received and to make any
improvements needed. If the course has been carefully prepared, feedback is
likely to be rewarding both for the Tutor and for the Facilitators.
Whatever the government policy may be regarding the award of a certificate of
competence, some record of attendance and level of competence reached by
each learner must be kept so that details may be checked later.
Learners' answers will yield valuable information on how useful they find this
type of training (a suitable questionnaire is provided in Annex 3). Allowing
learners to answer anonymously will encourage frankness.

Introduction to the course
The very first session with the learners in the meeting room must preferably
have seating in a semicircular arrangement as indicated in the diagram. If the
chairs do not have fixed supports for notebooks, it will be helpful to have
small desks or tables available.
The Tutor must first introduce himself or herself and write his or her name on
the board or flipchart, telling the learners a little about his or her background
and job. Each of the facilitators is then asked to do the same thing. The
learners introduce themselves next. It may be helpful to divide the learners into
pairs and ask them to exchange names, information about jobs, home towns,

etc. Each learner can then introduce his or her partner to the whole group. This
method often has the effect of reducing tension, and a relaxed atmosphere is a
good learning atmosphere.
The learners will have been given their copies of the Learner's Guide. After
10 minutes or so to read through the Introduction, the Tutor shall briefly deal
with the various topics covered, explaining, for instance, that working in small
groups with facilitators should make learning easier, and that there should be
little need to take notes during the course. It must be stressed that the course
will involve a great deal of exercises, since this is the best way to acquire the
necessary skills. The objectives of the various Learning Units must be
discussed so that the learners understand exactly what they should have
achieved by the end of the course. One important point is that the learners
must keep these objectives in mind throughout the course and always ask for
help if they feel uncertain of having achieved them. Each learner is likely to be
more aware than the facilitators of how well he or she has understood a
particular topic or has mastered a particular skill – it is the job of the
facilitators to make the learning process as effective as possible.

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Introduction

The learners must at that time be encouraged to discuss the training
programme – what they expect of it, what aspects are worrying them, and so
forth. The Tutor must explain that both Tutor and Facilitators will welcome
feedback throughout the course – constructive criticism from the learners will
help you improve the training programme. Other questions must be dealt with
at this time. The Tutor must finally explain that evaluation will be a
continuous process throughout the training course, stressing that pre and posttests are part of the learning experience and should be enjoyed rather than

feared. Their purpose is to allow an assessment of the learners' starting level,
to correct mistakes and clarify misunderstandings. The learners must read all
the questions (and any supplementary instructions) very carefully.

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Introduction to basic epidemiology & principles of simple statistics for tropical disease control – Tutor’s guide

.

16


Introduction to the course

Figure 1: Suggested timetable (to be adapted as circumstances dictate)*
Period
A.M.

Day 1
Introduction to the course and
how it will function
(40 minutes)

Day 2
Measures of central tendency
– mean, median, mode
(40 minutes)

Day 3
Survey: overview of methods
and steps (40 minutes)
Exercises: Survey (20 minutes)

Pre-test (60 minutes)
Introduction to epidemiology
(40 minutes)

Exercises: Measures of
central tendency
(40 minutes)

P.M.

Exercise: Survey – sampling
(90 minutes)

BREAK (20 minutes)

BREAK (20 minutes)

Exercises: Rates, ratios and
proportions, (80 minutes)

Exercises: Measures of
variability (40 minutes)
Exercises: Group
presentation, data
presentation (60 minutes)

Health facility-based studies
(60 minutes)

Principles of surveillance and the
evaluation of surveillance systems
(40 minutes)

Exercises: Survey – Analysis
(75 minutes)

BREAK (20 minutes)
Survey: sampling (60 minutes)

Measures of variability
(40 minutes)

Day 4
Survey: Analysis (75 minutes)

Sensitivity and specificity
(60 minutes 10 minutes)
BREAK (20 minutes)
Sensitivity and specificity
(80 minutes)

Survey: questionnaire design
(45 minutes)

Exercises Sensitivity and specificity
(75 minutes)

BREAK (15 minutes)
Evaluation of exercises on
surveillance (40 minutes

Exercises: Survey –
questionnaire (80 minutes)

BREAK (15 minutes)

BREAK (15 minutes)

BREAK (15 minutes)

Data presentation - tables,
graphs, and charts (40 minutes)

Exercises: Health facilitybased studies (105 minutes)

General discussion on basic
epidemiology and simple statistics
(30 minutes)

Exercises: Data presentation
(45 minutes)
Post-test (60 minutes)
* This 4-day schedule is an absolute minimum; a 5-day schedule allows for a less crowded approach and may be preferable

17

Introduction to the course

18


Introduction to epidemiology

Learning Unit 1

Learning Unit 1

Introduction to epidemiology
Reminder:
After 10 minutes or so to read through the Introduction, the Tutor shall briefly deal with the
various topics covered, explaining, for instance, that working in small groups with facilitators
should make learning easier, and that there should be little need to take notes during the
course. It must be stressed that the course will involve a great deal of exercises, since this is
the best way to acquire the necessary skills.
As the learning objectives state, this Unit is intended to help learners achieve the
following :
· Provide a definition of epidemiology
· Provide a definition of surveillance
· Define descriptive studies and describe their purpose
· Define analytic studies and describe their purpose
· Describe the major types of descriptive studies and their primary uses
· Describe the major types of analytic studies
· Provide a definition of random error, bias, confounding and validity

The learner's guide provides a definition of epidemiology and briefly describes the purpose of

various descriptive and analytic studies, but you should be particularly careful to check that
learners have understood the role of these studies for tropical diseases control. Stimulate
discussion; be particularly careful to explain any items that might be misunderstood, so that
misconceptions are eliminated from the outset.

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Introduction to basic epidemiology and simple statistics for tropical diseases control : Tutor’s Guide

20


Rates, ratios, and proportions

Learning Unit 2

Learning Unit 2

Rates, ratios, and proportions
As the learning objectives state, this Unit is intended to help learners
achieve the following :
§

Define the terms rate, ratio and proportion

§

Differentiate incidence rate from prevalence rate and give examples of their uses

§

Differentiate point prevalence from period prevalence

§

Determine the correct denominator for the calculation of each of these terms

§

Calculate rates, ratios, and proportions using appropriate numerators,
denominators and constants

§

Apply the concepts of rate ratios and rate differences

§

Apply the concepts of standardization of rates.

Describe briefly the terms mentioned above, the purposes for which these
measurements are used and their primary users. Illustrate your presentation
with simple examples.

Exercises on rates, ratios, and proportions
For conducting the exercise session it is best to have the learners in your class
work individually, although if some are particularly weak in mathematics, you
may wish to have them work in pairs with those who are more comfortable
with numbers. You and the facilitators may wish to circulate around the room

as they are working to help anyone who is having difficulties. For some of the
calculations, the learners may write their results in a slightly different way than
expressed here. For example, 9/100 can also be expressed as 90/1000. In
general, one should either follow convention (infant mortality is always
expressed per 1000 live births for example) or use the figure that leaves one
digit to the left of the decimal point for the smallest rate in a series.

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Introduction to basic epidemiology and simple statistics for tropical diseases control : Tutor’s Guide

Exercise 1
(a)

The number of cases has increased steadily, and in 1994 there were
more than twice as many cases as in 1990.

(b)

The rate per 100 is calculated by taking the number of cases divided by
the population and multiplying by 100.
Year
1990
1991
1992
1993
1994

Rate per 100

6.3
6.2
6.2
6.3
6.5

The rate has been relatively stable over the 5-year period, although it appears
to be increasing slightly in 1994.
(c)

While the number of cases has increased very rapidly, so has the
population. The rate, therefore, has remained relatively stable.

(d)

The rate, because it takes into account the changes in population size,
provides a more realistic idea of the amount of disease in the
population.

Exercise 2
(a)

The rate in province Z is 9.0/100, much higher than the rate of 6.6/100
seen in province X.

(b)

Although the disease rate is much higher in province Z, this province
has a much smaller population. A decision on allocation of funds will
depend on a variety of factors. If the goal of the malaria programme is

to prevent as many cases of malaria as possible, funds may be better
spent in province X, where the rate is somewhat lower but the
population and total number of malaria cases is much larger. One
should also find out if the rate in province Z is stable or is changing
rapidly.

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Rates, ratios, and proportions

Learning Unit 2

Exercise 3
(a)

47%.

(b)

A prevalence rate since it measures all cases of parasitaemia at the time
of the survey, some of which may have just occurred and some of
which are likely to have been going on for some period of time.

Exercise 4
(a)

Ratio = 49 140:23 250 = 2.1:1 (division of each figure by 23 250)

(b)

% males
% females

or

= [49 140/(49 140 + 23 250)] = 0.679 =
= [23 250/(49 140 + 23 250)] = 0.321 =
= 1-0.679 = 0.321 =

67.9%
32.1%
32.1%

Exercise 5
(a)

The age group 15 years and over.

(b)

The 1-4 year age group.

(c)

The age-group 15 years and over accounts for half the cases, but also
for over half the population. The group at greatest risk of getting
disease is not the 15 years and over, but the 1-4 year olds.
If you are planning hospital beds or ordering antimalarial drugs, the
percentage value may be more useful to you.

If you are deciding who is at risk for purposes of an intervention
programme, the age-specific incidence rate will be more useful.

23