EPIDEMIOLOGICAL STUDIES II (Cohort) Flashcards
a tenth of a roman legion: 300-600 soldiers
How the Roman Army was organized
Cohort (Latin) The legion was divided into ten cohorts Each cohort was made up of six centuries The centuries were commanded by a centurion Centuries originally had 100 men
Group of people who share a common experience or distinct set of characteristics during a defined time period
Cohort Studies
Cohort: Designate a group of individuals who are followed up over a period of time
Shared characteristics: secondary to common setting or common experience
Common setting: school cohort
Common experience: birth cohort
Also known as Follow-up studies, incidence studies, prospective studies, longitudinal studies, panel studies
Group or groups of persons are defined on the basis of presence or absence of exposure to a suspected risk factor of a disease
Cohort Study
Investigator starts with group of individuals apparently free from the disease of interest
Disease-free individuals then follow them up for a certain period of time and check for the presence of the outcome of the disease
Groups of individuals are divided into who are exposed and those not exposed to a suspected risk factor
Design: start with the exposed and does that are not exposed and followed during period of time
Followed during a period of time to determine and compare the occurrence of outcome among exposed and unexposed
Cohort Study
Outcome = disease
Well suited for assessing the effects of rare exposures
Ex: Cohort Study of Oral Contraceptive Use and Bacteriuria Among Women Aged 16-49 Years Old
Exposure: oral contraceptive use
Outcome: bacteriuria
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
General Feature of Cohort Study
- Proceeds from a suspected cause or etiological agent to the disease outcome
- Exposure is measured at the start of the study
C
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
General Feature of Cohort Study
- Not followed up to measure the outcome at the end of study
- Longitudinal type of study: 2 different time points
B
ODD MAN OUT:
Uses of Cohort Study
- Identify risk factors
- Identify protective factors against diseases
- Identify prognostic factors for outcome of disease
- Describe the natural history of disease
- Generate hypothesis in a descriptive approach
- Project incidence/ proportion/ number of new cases of disease over a period of time; data useful for planning acute rare services
- Assess effectiveness of preventive programs/ measures
- Generate hypothesis in a descriptive approach
Types of Cohort studies are based on:
Time relationship between initiation of study
Occurrence of disease
Cohort Study based on Time relationship between initiation of study:
it is based on concepts
Conceptual sense
Cohort Study based on Time relationship between initiation of study:
establishes causality; temporality
Temporal sense
Cohort Study based on Occurrence of disease:
exposure is measured first; establishes temporality
Exposure —> outcome
Prospective
Cohort Study based on Occurrence of disease:
outcome is measured first
Outcome —> exposure
Retrospective
Sources of Cohort Groups:
Special exposure groups
Special resource groups
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
Selection of Comparison Groups
- Groups being compared should not be similar to the study group in all respects except the exposure or determinant under experimentation
- Information obtained from the non-exposed groups is inadequate for comparison with the exposed group
D
Types of Comparison Groups:
used for general population cohorts
Internal comparison group
Types of Comparison Groups:
special exposure cohorts
External comparison group
Sources of Data
Pre-existing records
Direct physical examinations and/ or laboratory test of cohort members
Self reports of cohort members through interviews
Environmental measurements
Exposure Data
Sources of Data
Obituaries and death certificates
Periodic exams of the participants
Health records
Outcome Data
Measures of _______ :
Incidence proportion / cumulative incidence
Incidence rate / incidence density
disease occurrence
Measures of \_\_\_\_\_\_\_: Relative risk: Ratio measures Risk ratio Rate ratio Risk difference/ excess risk: Ratio measure
association
Measure of _______ ;
Difference measures: Attributable risk
Proportion of cases among exposed attributable to the exposure
potential impact
measure of disease occurrence
a + c x 100/a +b+c+d
CIE+: a x 100/ a+b
CIE-: c x 100/c+d
Cumulative Incidence
Cumulative incidence/ incidence proportion
measure of association
RR or CIR= CIE+/CIE-
Relative Risk Risk ratio (RR) or Cumulative Incidence ratio (CIR)
TRUE OR FALSE:
1 is the null value of Relative Risk
TRUE
No association between exposed and unexposed groups
if RR is > 1: harmful association; incidence in exposed is higher
if RR is < 1: beneficial association; incidence in unexposed is higher
0 is null value
RD = CIE+ - CIE-
Risk Difference or excess risk
Risk difference RD or Cumulative Incidence Difference CID
% attributable risk = CIE+ - CIE- /CIE+
Attributable Risk
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
Advantages of Cohort Study
- Possible to compute for the relative risk
- Yield information on the incidence of the disease (New cases of diseases)
C
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
Advantages of Cohort Study
- Temporal relationship between exposure and disease is not clearly defined
- Design is particularly inefficient for studies involving rare exposure factors
D
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
Advantages of Cohort Study
- Strongest observational design for establishing cause-effect relationship
- Free of random and systematic errors
A
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
Limitations of Cohort Study
- Time consuming; follow up is made
- Often a small sample size
A
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
Limitations of Cohort Study
- Cheap: takes only a few days depending on the study
- Not efficient for the study of rare diseases: less exposure
B
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
Limitations of Cohort Study
- Losses to follow-up may diminish validity: you may lose respondents
- Changes over time in diagnostic methods may lead to biased results
C
Sources of Bias:
Selection bias
Misclassification bias: differential and non-differential
Confounding
