Prospective Epidemiological Studies: Risk & Incidence
Prospective Epidemiological Studies
Prospective (Follow-Up) Study:
- A type of cohort study which analyzes risk factors and follows a group of people who do not have the disease, using correlations to determine the absolute risk of subject contraction.
- Prospective = cause → effect
- Cohort = group of people who share a common characteristic or experience within a defined period (e.g., are born, are exposed to a drug or vaccine or pollutant, or undergo a certain medical procedure).
- Two groups of patients are followed:
One group (the control) does not have the investigated risk-factor for the studied disease. The second (exposed) does have the risk factor.
- The comparison group may be the general population from which the cohort is drawn, or it may be another cohort of persons.
- The occurrence and incidence of the disease is followed for a given period of time, and an association can then be made between the initial presence of the determined risk factor and the incidence of the disease.
The Groups:
Initially – two groups:
- Exposed (a + b)
- Unexposed (c + d) = Control
Later – four groups will emerge:
From the exposed patients, two outcomes are possible:
- Development of the disease (a)
- No disease (b)
From the control group, two outcomes are also possible:
- Development of the disease (c)
- No disease (d)
Characteristics:
- Outcome is measured after exposure.
- Yields true incidence rates and relative risks.
- May uncover unanticipated associations with outcome.
- Best for common outcomes.
- Expensive.
- Requires large numbers.
- Takes a long time to complete.
- Prone to attrition bias (compensate by using person-time methods).
- Prone to the bias of change in methods over time.
Calculations:
- Overall Incidence Rate
- Incidence in the Risk Group (Ir) is determined by the number of patients in whom the disease developed (a) divided by the initial total number of risk patient (a + b).
- Incidence in the Control Group (Ic) is determined by the number of patients in whom the disease developed (c) divided by the initial total number of control patients (c + d).
Relative Risk (RR)
- RR = 1 – no association between the exposure and the risk
- RR >1 – the exposure is a potential risk factor for the disease
- RR <1 – the exposure is potentially protective for the disease
Attributable Risk (AR)
This is an absolute measure with the same units as the incidence rate (e.g. per person per year).
It indicates the number of cases of the disease among the exposed group that could theoretically be prevented if the exposure were completely eliminated (assuming the risk is causal).