DescriptiveEpidemiology

Dr. Dalia El-ShafeiAssistant Professor, Community Medicine

Department, Zagazig Universityhttp://www.slideshare.net/daliaelshafei

Epidemiology is derived from the Greek,

Epidemiology is the basic science

of Public Health

Definition of EpidemiologyThe STUDY of the DISTRIBUTION & DETERMINANTS of HEALTH-RELATED STATES in specified POPULATIONS, and the application of this study to CONTROL of health problems."

Is the basic science of public health

Provides insight regarding the nature, causes, and extent of health & disease

Provides information needed to plan & target resources appropriately

So, Epidemiology

Study design

Non-experimental “Observational” studies:-Investigator measures but does not intervene.-The investigator observes natural course of events, observing who is exposed & who is not, who is diseased & who is healthy.-The non-experimental studies can be either descriptive or analytical.

Experimental “Interventional” studies: -Active trial to change disease determinant by the investigator who controls the exposure. -Investigator allocates the exposure & follows the subjects. -Participant are identified on the basis of their exposure status & followed to determine whether they develop the outcome or not.

Epidemiological studies

Observational “Non-experimental”Descriptive Population

Correlational studies

IndividualsCross sectional surveys

Case reports

Case series

Analytical Comparative cross-sectional

Case-control

Cohort Prospective

Retrospective

ExperimentalClinical trials

Community trials

Field trials

Descriptive epidemiolog

y

Descriptive Epidemiological studies

To Know the situation: (what is the problem? What are its

manifestations?)

Or

To Describe the general characteristics of a disease /or health problem in relation to PPT

(Person– Place –Time).

□ Person: Who is getting sick?□ Place: Where is the sickness

occurring?□ Time: When is the sickness

occurring?

“PPT = Person, Place, Time”

Observational “Non-experimental”Descriptive Population

Correlational studies

IndividualsCross sectional

Case reports

Case series

Analytical Comparative cross-sectional

Case-control

Cohort Prospective

Retrospective

ExperimentalClinical trials

Community trials

Field trials

&Case Series

1- Case Report:

Example:

Intestinal obstruction was reported in a young child.. Documents showed that this child received Rota virus vaccine 3 months ago. A detailed report about this unusual event & exposure was published in a medical journal. The investigator formulated a hypothesis that Rota virus vaccine may have been responsible for the rare occurrence of this event.

Features of the Case Report:

It consists of a careful & detailed report (published

in medical journals) by one or more clinicians of

unusual medical condition.

It represents the 1st clue in the identification of a new

disease.

It leads to formulation of a new hypothesis.

2- Case Series:It is the only study which depends on Routine Surveillance.

Example of the case series study:• During 1950 , 8 cases of cancer lung were admitted to different hospitals during the same period of time. Taking history from these patients showed that they were miners. This unusual circumstance suggested that the miners may been exposed to something. Investigating this circumstance showed high concentration of radon gas. A hypothesis was formulated that lung cancer is related to exposure to radon.

The benefits of case report & case series:They identify a new case and/ or an unusual variation of a disease occurrence.

• Formulate a new hypothesis for disease occurrence.

• Trigger “stimulate start of analytic studies to be conducted to identify risk factors of disease”.

• Modification of case series to be a case-control study by using a comparison group.

The limitations of the case report & case series:

o For the case report, the presence of any exposure may be coincidental because it is based on a single experience .

o Lack of the comparison group in case series can either obscure the relationship or suggest an association which is not actually exist.

o Both of them cannot be used to show the causal association i.e. can not be used to test the hypothesis.

Correlation study: ( Ecological study)

The source of data is the entire population . It compares disease frequencies:

- between different population during the same period of time Or - In the same population at different in time .

It compares 2 quantitative variables.

Correlation between one of climatic indicator (Temp.) & frequency of cerebrovascular storks.

Figure 1 shows the correlation between the average regional temp. & the frequency of CVSs in different countries. Countries with the highest average temp. have the highest rates of CVSs and vice versa.

100 The average regional temp. & the frequency of CVSs

80

+ve Correlation (r = +1) 60

40

20

0 10°C 15°C 20°C 25°C 30°C 35°C 40°C

The Average Regional Temperature

Example 2:

The average number of mammography carried for

women above 50 years of age per year & the

mortality from cancer breast.

This can be presented by the following figures.

The average number of mammography per year for woman

above 50 & the mortality from cancer breast

100 Negative Correlation (r = -1)

80 60

40

20

0 3 4 5 6 7 8 9

The average number of mammography per year for woman above 50

The advantages of the correlation study:

1- Formulates new hypothesis.

2- Quick & Cheap.

The limitations of correlation studies:1. As the value of exposure is quantified by the average, it is impossible to link the exposure & the disease in a particular individual. It is not possible to tell that the person who gets cerebro-vascular stroke is the one who is exposed to high temperature.

2. They cannot be used for testing the hypothesis.

3. Lack of the ability to control for the effects of the confounding factors.

Confounding factors:

These are factors other than the studied one that disturb the relation between the studied exposure and the disease of interest. For example: The association between the average family size and the frequency of iron deficiency anemia may be due to other factors such as the pattern of diet, the infectious diseases , the socioeconomic conditions and parasitic infections.

Impacts of the Confounding Factors

Large Family size (Exposure)

Iron deficiency anemia (condition)

Parasitic InfectionPattern of Diet

Mothers AwarenessMothers Education

(Confounding factors)

Cross sectional study (Prevalence study):

Population Sample

Without Exposure & without disease

Without Exposure & with disease

With Exposure &without disease

With Exposure & with disease

The uses of cross-sectional study:

Estimation of prevalence rate of disease or any health related phenomena.

It leads to formulation of hypothesis. It is suitable for chronic diseases with long latency.

Quick & cheap, compared to case-control & cohort studies.

The Limitations of the cross-sectional study: Can’t be used to test hypothesis (chicken-egg dilemma). Deals with survivals only but those who died, cured or migrated are not included. Can’t be used in acute diseases of short duration. Not suitable for rare diseases (Compared with the case control study).

Example:

During the year 2004 , a representative sample of secondary school pupils in a city x (n=400) were asked about consumption of high caloric diet & examined to detect obesity. Questions:Draw the flow chart.Tabulate the data.Write the title of the table.

No consumption of high Caloric diet without obesity

n=304

No consumption of high Caloric diet with obesity

n=16

Consumption of high Caloric diet without obesity

n=60

Consumption of high caloric diet with obesity

n=20

Secondary school pupils

Samplen=400

The flow Chart:

Distribution of the studied sample of secondary school pupils in the city X during the year 2004 according to

consumption of high caloric diet & obesity.

Consumption of high caloric diet

With obesity

Without obesity

Total

Yes 20 60 80No 16 304 320

Total 36 364 400

Prevalence of obesity among those consumed high caloric diet (P1 ) =

20 X 100 = 25%80

Prevalence of obesity among those don’t consume high caloric diet (P2) =

16 X 100 = 5%320

The prevalence rate =

The total number of all cases (old and new) in certain area at a given time X 100 The total number of population in the same area and time

Population Village (Sample)

No HCV, no Bl.T (D) (E)

HCV, Bl.T.(D) (E)

No HCV, Bl.T.(D) (E) HCV, no Bl.T.

(D) (E)

Hepatitis C infect. & Blood Transfusion

4 different outcomes

Cross-Sectional Design

Population Village (n= 200)

Not D, not E 120

Not D, E 40

D, not E 10

D, E 30

Descriptive Data: Prevalence RatePrevalence of HCV in village: 40/ 200 = 20%Prevalence among males, among females….Prevalence among different age groups…….Prevalence of blood transfusion: 70/200 = 35%

Prevalence of HCV among those receiving blood transfusion 30/70 = 42.8%

Prevalence of HCV among those not receiving blood transf.: 10/130 = 7.6%

Exercise1:-Description of 35 patients with thyroid cancer are regarding past history of exposure to radiation and response to surgical treatment

Feedback:-

Case series

Exercise 2:-A 39-year old man who presents with mild sore throat, fever, malaise and headache was treated with penicillin for presumed streptococcal infection. He returned after a week with hypotension, fever and

abdominal pain .A diagnosis of Rocky Mountain spotted fever was

made and he responded good to chloramphenicol.

Feedback 2:-

Case report

Exercise 3:- 500 patients were classified according to their

body mass index (obese or not) and simultaneously according to having knee osteoarthrosis

Feedback of Exercise 3:-

Cross sectional study