Dr. Hazem El Ajrami Master Degree in Orthodontic & Pedodontic Al Azhar university Lama K El Banna

2. College of Dentistry
Community Dentistry
Epidemiology
Dr. Hazem El Ajrami

3. • Epidemiologic investigation should
follow the following scientific pattern:
I. Establishing the objectives.
II. Design of the investigation.
III. Selection of the sample.
IV. Conducting the examinations.
V. Analyzing the data.
VI. Drawing the conclusions.
VII.Publishing the report.

4. III. Selection of the sample:
When designing a study it is usually
impossible to examine every individual
in the population or universe under
investigation as resources in terms of money,
time and man power are not available for the
collection and analysis of such vast amounts
of data. For this reason, a small number of
individuals or a sample must be chosen from
the population.

6. A. Definition :
 Sample is a part of population or set of things
which we actually does study.
 As it is impossible to examine every
individual in the population (no sufficient
resources), so we must chose a sample from
the population which express the condition in
this population.

8. B. Advantages of sampling:
I. Saving time , effort and money.
II. Allow performance of the study when the
available resources are limited.

9. C. Sample types:
1) Selected sample:
Is one in which a criterion is set for the
inclusion of each individual in the study, and
each individual satisfies this criterion will be
included in this sample.
2) Random sample:
This technique will provide more valid
data from a population. For the sample to be
truly random, each individual must have an
equal chance of being included in the sample.
One of the easiest way of doing this is to use
random number tables.

11. • The basic procedures in such sampling are:
To prepare a sampling frame (i.e. a list
showing all the units from which
the sample is to be selected and it is
arranged in any order).
To decide the size of the sample.
To select the required number at random.

13. 3) Stratified random sample:
If the condition under investigation is known
to be related to various factors such as age,
sex, or area of residence, the population is first
divided into these groups (strata) and a
random sample taken within each group
(stratum). For example, dental caries is an age-
specific disease and so any population on
which a survey is to be done for detecting the
prevalence or extent of the disease should be
stratified by age.

17. 4) Cluster sample:
In some cases, it is more convenient for
administration and economic reasons to
sample from clusters rather than from
individuals. In cluster sampling, a simple
random sample is selected not from
individual subjects but of groups or clusters
of individuals. The clusters may be schools,
villages .. etc.

19. D. Sample size:
The sample size is determined according to:
1. The available resources, facilities and time factor.
2. Prevalence of the disease, the lower the prevalence,
the bigger the sample size and vice versa.
3. Number of variables and subgroups.
4. Expected range of differences in the characteristics
under study. e.g. → age, income and educational
level. The smaller the range, the smaller the sample
size and vice versa.

20. IV. Conducting the examinations:
For the scientific study of any dental
disease and conditions, three aspects are of
great importance, the examination methods
and diagnostic aids, the diagnostic criteria,
and the indices used for measuring and
reporting. Basic requirements for the mouth
examination are a chair, preferably with a
head rest, on which to seat the subject, a
source of illumination, and some method of
cleaning the teeth to remove loose debris.

22. Dental Caries
The criteria for caries diagnosis should be
defined before starting the examination. The
commonest definition postulates that a tooth is
considered carious when a sharp explorer catches
in a cavity with a detectably soft floor and/or
some undermined enamel or a breakdown in the
walls of a pit or fissure. For numerical evaluation
of caries in a group of population, certain
measures or indices had been devised.

24. • An index:
It is defined as a numerical value describing
the relative status of a population on a
graduated scale with definite upper and lower
limits designed to facilitate comparison with
other population classified by the same criteria
and method. An index describes the prevalence
of a disease in a population and also describes
the severity or intensity of the condition.

25. • Requirements of an ideal index:
An ideal index should possess the following
characteristics:
1. Clarity: The criteria of scoring should be
clear and easy to understand and apply in
practice.
2. Objectivity: Each index should have a
specific and clear cut objectives.

26. 3. Simplicity: Each index should be easy to
apply even in field studies.
4. Reliability: The index should be reliable so
that when a condition in the same subject is
measured repeatedly, it should give the same
results. There should be no intra-examiner or
inter-examiner variation.
5. Sensitivity: The index should be able to
detect a reasonably small difference in what is
being assessed.

27. 6. Acceptability: The index should be acceptable
to the subject. In other words it should not
cause pain or discomfort to the subject.
7. Amenability to evaluation: The index should
be amenable to statistical evaluation.
8. Economical: The index should be economical
in field studies.
9. Not time consuming: The index should not be
time consuming so that it can be used in a
large number of subjects.

28. Thank You