ANALYTICAL STUDIES

ANALYTICAL STUDIES Prospective Studies COHORT Prepared by: Dr. Sahar Sabbour Community Medicine Department

Points discussed • Types of analytic studies • Aim of analytic studies • Flow chart of the design • Types of cohort studies • Analysis of results • Examples from the literature • Advantages & disadvantages

Intended Learning Outcomes Students should be able to: • List types of analytical studies • Define cohort, identify types of cohort studies • Describe the prospective approach • Investigate problems using the prospective design • Draw a flow chart showing the cohort study • Compare between cohort and case control studies • Calculate rates from cohort studies

Analytical Studies Introduction: Analytical studies are either: • ObservationalCase-Control Cohort Study • Experimental(Intervention): Animal Experiments Human Therapeutic trials Preventive trials

Analytic Studies • Analytic studies, etiologic studies, are performed to test specific hypothesis about a specific health problem. • In general, associations observed in descriptive studies are often the basis for gathering more specific data and testing hypothesis in additional studies.

Analytic studies involve the selection and comparison of two or more groups of persons, based on either their exposure or disease status…. WHY? • To evaluate an association between exposure and disease. • Analytic studies focuses on the magnitude of the association between the exposure and the health problem under the study.

A fourfold table Retrospective (Case-Control) cases controls EXPOSURE DISEASE present absent a b Prospective (Cohort) present exposed d c absent Not exposed Mausner, 1985 Total Total

The difference between the two types of studies lies in the way the study groups are assembled • With either method of study, if there is a positive association between the factor and the disease: • Those exposed will tend to develop the disease (group a), • Those not exposed will tend not to develop it (group d).

The Prospective Approach • The general concept of a prospective study is relatively simple. • This type of study has been described by a variety of items: -Cohort -Incidence -Longitudinal -Forward looking -Follow-up

Prospective Cohort “concurrent” • COHORT Study Retrospective Cohort “non-concurrent” Historical prospective

The Prospective Approach(cont.) • 1. It starts with a group of people (a cohort) all considered to be free of a given disease. • Information is obtained to determine persons having aparticular characteristic(certain exposure) that is suspected of being related to the development of disease being investigated. • 2. These individuals are thenfollowedfor a period of time to observe who develops/or dies from that disease • 3. Incidence or deathratesfor the disease are then calculated.

The Prospective Approach(cont.) • 4. Rates are compared for those with the characteristic and those without it. • 5. If the rates (of development of disease) are different, an association can be said to exist between the characteristic (exposure) and the disease. • 6. It is important to obtain information on other characteristic of the study groups: age, sex, … to account for an influence of any factors related to the disease.

What is a cohort ? • A cohort is a group of persons who share a common experience within a defined time period. Example: • Birth cohort, marriage cohort, occupational cohort

Cohort Study (cont.) Essential points: • Exposed individuals in the study should be representative of all exposed persons. • Unexposed persons should be representative of all unexposed persons in the population.

Cohort Study (cont.) Selection of Cohorts: several approaches • Accessible group (volunteers) • Group with available records/history of exposure • Group experiencing some particular exposure (arising during work) Cohorts may be heterogeneous or homogenous

Heterogeneous: with respect to some previous exposure as study of lung cancer and smoking. • Homogenous in exposure: As study of the frequency of cancer among asbestos workers. The comparison group is the general population values Demonstrate excess in deaths among asbestos workers.

Cohort Study (Prospective Design) Passive smoking & respiratory infections in children • Is passive exposure to tobacco smoke associated with increased respiratory infections in children ? • Design: Children exposed and not exposed tobacco smoke in their homes Follow them in time for disease occurrence.

Start Outcome Family smoker 500 children Exposed Diseased 300 Children (<12 yrs) 1000 Not diseased 200 1 year Diseased 120 Family non-smoker 500 children Not exposed Not diseased 380

Rate:Incidence rate • Incidence of Resp. Infection among exposed children: 300 • 500 = 60% • Incidence of Resp. Infect. Among non exposed children: 120 • 500 = 24%

Cohort Study(cont.) Relative Risk: Incidence rate among exposed Risk Ratio Incidence rate in non exposed. 60 24 = 2.5 Relative Risk is adirect measure of risk(to assess the etiologic role of a factor in disease occurrence). 300 x 500 500 120

Cohort Study(cont.) Relative Risk: Smoking • Lung Cancer mortality: RR=18.57 • Myocardial infarction mortality: RR=1.35 It measures the strength of association

Cohort Study(cont.) Attributable Risk: The absolute difference inIncidence rates among groups. “Risk Difference” RD 60 - 24 = 36% The extent to which the incidence of disease can be attributed to the risk factor Smoking -Lung cancer mortality: RD=1.23 -Myocardial infarction mortality RD=1.25

Doll and Hill study : Mortality of British doctors cited from Mausner, 1985

The previous table suggests that prevention of coronary heart disease would require alteration of other factors in addition to smoking. • The population attributable risk: relates both relative risk and frequency of the factor in the population • i.e. a large proportion of the deaths from lung cancer in the total population are due to smoking not only because of the high RR associated with smoking, but also bec large proportion of the pop that smoke.

Examples from the literature • Framingham Heart Study initiated in 1948 by US Public Health Services: to study the relationship of a variety of factors to the subsequent development of heart disease Group of persons 30 – 62yrs 6,500 Both sexes 20 years follow up outcome Information: S. cholest.level Bl.pressure , weight Cig. Smoking

Occupation Based Studies to study effect of exposures • Benzene workers and Leukemia • Coke-oven workers and lung cancer • Asbestos workers and lung cancer • Radium dial painters and oral cancer

There is an increasing risk of CHD with increasing initial Serum cholest. Levels in the 45-54 age group from a relative Risk of 1.13-3.25 M, 1.13-2.89 F

Advantages of Cohort Study • Correct classification of exposure before disease develops. • Permits calculation of incidence rates thus, a direct measure of relative risk, and attributable risk. • Many possible outcomes to the same exposure can be studied. • No chick egg dilema • Accurate

Disadvantages of Cohort Study • Large number of people are needed (large scale). • Time consuming (follow up) • Losing people in follow up(Attrition) • Expensive • Status of subjects may be changed leading to error in classification of exposure eg. Change in habit, occupation. • Administrative problems: loss of staff, funding, high costs of the extensive record keeping

Non concurrent studies Retrospective Cohort • The period of observation starts from some date in the past. • They usually involve specially exposed groups or industrial populations. • Done by using company records of past & present employees: • Information: - date of employment - date of departure - duration, degree of exposure - status: living/dead

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ANALYTICAL STUDIES