Methodology

METHODOlOGYEPIDEMIOLOGICAL STUDY DESIGN

Dr. Sijal Fadhil Farhood AL-joborae
F.I.C.M.S (Baghdad)
M.Sc. Community (Nahrain)
M.B.Ch.B (Babylon University)

INTRODUCTION

The primary concern of epidemiologist is to study disease occurrence in people ,who during the course of their lives are exposed to numerous factors and circumstances, some of which may have a role in disease aetiology.
Unlike the clinician or the laboratory investigator, who is able to study the disease condition more precisely ,the epidemiologists employs carefully designed research strategies to explore disease aetiology.

CLASSIFICATION OF EPIDEMIOLOGICAL STUDIES

1- descriptive studies.
A-case reports and case series.
B-correlation studies.
C-cross sectional studies.
2-Analytic studies:
A-observational studies:
1-case control study.
2-cohort study.
B-interventional (experimental)studies:

DESCRIPTIVE EPIDEMIOLOGY

Describes the pattern of disease occurrence in terms of person, place, time model or host , agent ,environment model.
Defines the relationships of disease to the population at risk.

ADVANTAGES OF DESCRIPTIVE STUDIES

• 1-They use available data, so there is less time, less effort and money.
• 2-Describe disease patterns, so importance include
• -For the epidemiologists : They can identify risk factors and formulate research questions.
• -For the health administrators: identify population at risk (whom & where) to allocate resources and preventive programs.

Correlation or Ecological Studies

They are based on aggregate measures of exposure and outcome from several populations.
The population is the unit of observation available for study.
Eg: there is a positive correlation between fat consumption and breast cancer in many nations.
Ecological studies may be more appropriate than other designs when studying the impact of an exposure on a community level.


Advantages
1-Quick & easy as it employs available data.
2-It is the first step in investigating a possible exposure disease relationship.
Disadvantages
1-It cannot link exposure to disease in individuals (unknown temporality)
2.correlational data represent average exposure level rather than actual individual values.
3-It cannot control confounders.

Case report & case series report:

case report: The individual is the unit of observation available for study.
Clinical case with “unusual” clinical picture
• They describe the experience of a single patient or a small number of patients with a similar diagnosis reflecting unusual features of a disease.
• They help in:
• -Formulation of a hypothesis suggesting an etiological association
• -Represent the first clues in identification of new disease or epidemic.

Case series

First case report may stimulate compilation of additional case reports….a case series or(are collection of individual cases report occurring within a fairly short period of time.

Advantages:

• 1-Use available clinical data
• 2-Detailed individual data
• 3-Suggests need for investigation (hypothesis generation)

Disadvantages:
1-May reflect experience of one person or one clinician
2-No explicit comparison group
3-Can not be used to test the presence of a valid statistic association.

Cross sectional (prevalence)study

• 1-Collection of data on several individuals at “one point” in time.
• 2-Determines prevalence at a point in time
• 3-Therefore, CS is a prevalence study
• 4-The exposure and disease status are assessed simultaneously among individuals in a well defined population.
• 5-Snapshot in time

Example

We are interested in the possible relationship of
increased serum cholesterol level (the exposure) to
electrocardiographic (ECG) evidence of CHD (the
disease). We survey a population; for each participant
we determine the serum cholesterol level and
perform an ECG for evidence of CHD. This type of
study design is called a cross-sectional study because
both exposure and disease outcome are determined
simultaneously for each subject; it is as if we were
viewing a snapshot of the population at a certain
point in time.


If we determine in such a study that there appears to be an association between increased cholesterol level and CHD, we are left with several
problems. First, in this cross-sectional study, we are identifying prevalent cases of CHD rather than incident (new) cases; such prevalent cases may not be representative of all cases of CHD that have developed in this population. For example, identifying only prevalent cases would exclude those who died after the disease developed but before the study was carried out. Therefore, even if an association of exposure and disease is observed, the association may be with survival after CHD rather than
with the risk of developing CHD. Second, because the presence or absence of both exposure and disease was determined at the same time in each subject in the study, it is often not possible to establish a temporal relationship between the exposure and the onset of disease.

it is not possible to tell whether or not the increased cholesterol level preceded the development of CHD. Without information
on temporal relationships, it is conceivable that the increased cholesterol level could have occurred as a result of the coronary heart disease, or perhaps both may have occurred as a result of another factor. If it turns out that the exposure did
not precede the development of the disease, the association cannot reflect a causal relationship.
Consequently, although a cross-sectional study can be very suggestive of a possible risk factor or risk factors for a disease, when an association is found in such a study, given the limitations in establishing a temporal relationship between exposure and outcome, we rely on cohort and case- control
studies to establish etiologic relationships.

Advantages of cross sectional study

Provides information on frequency and characteristics of the disease
Standardized data collection tool.
Able to focus data collection in specific locations or specific groups of persons.
May make comparisons among study participants.
Relatively quick to do.
May be repeated to get data on trends.

Limitations

Inability to determine the temporal relationship between exposure and disease.
May be biased by lack of participation
Reflects prevalent, not incident cases and thus results may be related to duration of disease, or survival with disease


DESIGN

Disease No disease

a
b
C
d
a
b
c
d
a
b
c
d
exposed
Not
exposed
Disease
No
disease
Disease
No
disease
exposed
Not
exposed
exposed
Not
exposed






To determine weather there is an association between exposure and disease we have two choices:
1-We can calculate the prevalence of disease in persons with the exposure(a/a+b),and compare it with the prevalence of disease in person without the exposure(c/c+d).
2-or we can compare the prevalence of exposure in persons with the disease(a/a+c),to the prevalence of exposure in persons without the disease(b/b+d)

summary

In cross sectional study:
The exposure and disease status are assessed simultaneously among individuals in a well defined population.
It provide information about the frequency and characteristics of a disease by furnishing a “snapshot”.