Lecture 7: Cohort Studies

Cohort Study

The observational study most similar to a clinical trial is the ___ ___

Experimental Study

Type of analytic study

the investigator manipulates the conditions that give rise to the outcome under study

Clinical trials

Observational Study

the investigator does NOT manipulate the conditions, but rather observes the process

cohort studies, case-control studies, cross-sectional studies

disease free

In a cohort study, the cohort is ___ ___ at the start of follow-up and at-risk of getting the disease

before

In a cohort study, an individual's exposure status is observed/recorded ___ the onset of disease

Data collection is under your control, temporality of exposure and disease is well-established

Advantages of a prospective cohort study

More costly and time-consuming

Disadvantages of a prospective cohort study

Less expensive and less time consuming

Advantages of a retrospective cohort study

rely on data that was not compiled for the purposes of your study

Disadvantages of a retrospective cohort study

Retrospective cohort study

the exposure and outcome occur prior to the beginning of the study

exposure status

Ideally, only ___ ___ should differ between exposed and unexposed groups

Unexposed persons should be sampled from the same or comparable population

active follow-up

periodic, direct assessment of subject status through contact by mail, telephone, interview of clinical examination

passive follow-up

periodic, indirect assessment of subject status through the use of information collected for other purposes

of new cases of disease during follow-up / # of disease-free, at-risk subjects at the start of the follow-up

Risk equation

(risk in exposed)-(risk in unexposed)

Risk difference equation

(risk in exposed)/(risk in unexposed)

Risk ratio equation

RR=1

no association between exposure and disease RR (null value)

RR>1

harmful exposure (RR)

RR

protective exposure (RR)

The risk of disease in the exposed is RR times the risk of disease in the unexposed during study follow-up

Risk Ratio Interpretation

Risk Difference

The 'additional' or 'excess' risk of disease among the exposed compared with the unexposed

-1

Range of Risk Difference

RD=0

no association between exposure and disease, null value (RD)

RD > 0

harmful exposure (RD)

RD < 0

protective exposure (RD)

etiology

ratio measures are primarily used to learn about the ___ of a disease

difference measures

can tell us about etiology but are ALSO used to determine public health importance of a disease --> how much disease might be eliminated if the exposure was prevented?

Nearly impossible to follow the entire population for the specified time interval

What is the main problem with using risk?

People move in and out, follow-up time can be very long, high loss to follow-up due to high mortality

of new cases of disease during follow-up / total person-time contributed by subjects during follow-up

Rate equation

Can study several diseases

Useful for examining rare exposures (seek out those with exposure)

Calculate measures of disease frequency and association

Temporality

Advantages of Cohort Studies

Lost of subjects can introduce bias

Inefficient for rare diseases or disease with long latency/induction

Costly and time-consuming

Disadvantages of Cohort Studies