Population Cohort Studies Flashcards
What is a cohort?
a group of participants you follow over a period of time - study the natural progression of a disease
What are the strengths of cohort studies?
•Less biased information on risk factors because the risk factor is measured before the disease occurs
–Unlike case-control studies, which ascertain exposure retrospectively
•The results are easier to generalise to the population (external validity)
What are the weaknesses of cohort studies?
•Even for common diseases, a lot of follow-up is needed to detect a worthwhile number of cases
–Non-response, migration and loss-to-follow-up biases
- Impractical for rare diseases
- And for diseases which have a long pre-clinical phase
–The phase before the first clinical manifestations
what is the ‘relative risk ‘ ?
•incidence in those exposed to the factor divided by the incidence in those unexposed
1)It means that if the incidence is lower in those exposed to the factor, the RR will be less than 1. And if the incidence is higher, the RR will be greater than 1. If the incidence it the same in both groups then the relative risk will be 1!
What is the ‘survival time’ ?
•the length of time from entering the study to developing the end-point event
1) Survival time and survival analysis is a term that is used in cohort studies! It can be applied to any study in which people are followed from a defined starting point… and where the occurrence of a defined event (disease) is recorded (so time to death, time to time to recovery, time to complete wound healing )
what is ‘censoring’ ?
•the survival time on some subjects may be unobtainable (lost to follow up)
data that could not be obtained because of drop out or lost to follow up. The time from entering into a study to becoming censored it named ‘ censored survival time’ .
What is the ‘person-years’?
the observation time per person measured in years
the time that the person is observed in the study (in years).. So that includes those who do not achieve the end of the study because they drop out
what are the 3 components of a population cohort study?
•Defined population group of ‘healthy’ individuals
–Defined as not already having the disease or disorder(s) of interest
•Classified into groups depending on exposure status
–Exposure to factors that modify their risk
•Followed up to assess defined endpoints
Cohort studies provide strong evidence of what?
they provide strong evidence that the association between a risk factor and an outcome is causal - b/c the risk factors are measured before the disease occurs
