ch 8 - cohort study

Question 1

what is a cohort study?

Answer 1

• subjects are defined according to their exposure levels and followed for disease occurrence
• typically examines multiple exposures in relation to a disease; subjects are defined based on their disease status, and exposure histories are compared.
• the observational equivalent of experimental studies but researchers do not allocate exposure but locate natural experiments to observe the relationship between exposure and disease

Question 2

what are the three types of populations studied? what calculation is used to find them?

Answer 2

1. open dynamic- changeable - IR
2. fixed- loss may occur - IR
3. closed - no loses - ci

Question 3

what are exposed populations based on? what are the two types?

Answer 3

• depends on hypothesis under study, exposure frequency, feasibility
• general and special cohort

Question 4

general cohort

Answer 4

• investigates the exposures that are common with a population - alochol use
• researchers can study the effects of multiple E on one or more outcomes
• selected by professional groups or residents from a well-defined grographic area
• ex: framingham study

Question 5

special cohort

Answer 5

• investigates unsual and rare expsoures
• researchers can study the effects of a SINGLE exposure on one or more outcomes
• must go to where the expsoure has occured , will occcur

Question 6

counterfactual ideal

Answer 6

• the concept referring to what would have happened to an individual in the absence of the exposure
• The counterfactual ideal is hypothetical since, in reality, the exposed and unexposed study participants cannot exist simultaneously.
• The rationale of behind the counterfactual ideal is the notion that the comparison group and the exposed group are exactly the same except for the exposure

Question 7

dose-response relationship

Answer 7

risk of disease increases as intensity/duration of expsoure increases

Question 8

how do epidemiologist select sets of poeple?

Answer 8

• as similar as possible with respect to:
  other factors that can influence outcome
  to collection of comparable and accurate information

Question 9

what are the three types of comparison groups?

Answer 9

1. internal- unexposed members of the same cohort
2. general pop- based on preexisting pop data on disease incidence and mortality
3. comparison - consists of members from another cohort

Question 10

Internal comparison group? strength? weakness?

Answer 10

• are comprised of unexposed members of the same cohort
• is most comparable to the exposed group
• difficult to identify

Question 11

general pop? strengths and weakness

Answer 11

• maybe used as a comparison group when it is not possible to identify a comparable internal comparison group. and occurs in occupational cohort studies
• Accessible/stable data
• weakness:
  lack of comparison with the exposure group
  results may suffer from healthy worker effect
  data on key variables may be missing
• herbicide factory works vs general pop

Question 12

what is healthy worker effect?
why is it an issue?

Answer 12

• in occupational cohort studies the rates of morbidity and mortality among working population is lower than those in general population
• since its a health requirement for workers to be healthy
• issues is that it creates an underestimation of teh risk when comparing a worker to general popualtion

Question 13

comparison group?
strength
- weakness
- example?

Answer 13

• comprised of unexposed study participants from another population
• fairly comparable
• results can be difficult to interpret
  because comparison cohort often has other exposure
• comparing landscapers with construction workers

Question 14

strengths and weakness of compairson groups

Answer 14

• fairly comparable
• results often difficult to interpret because a comparison cohort has other E

Question 15

what are the three sources of exposure information?

Answer 15

• preexisting records
• questionnaire , interview
• direct testing

Question 16

how do we measure frequency and assoiation?

Answer 16

2x2 table (not realistic, in realiity we use statistical package)
CI or IR of E
RD or RR compairing E

Question 17

what are strength of cohort studies?

Answer 17

• clear temporal sequence between E and D especially if prospective
• efficient for rare E
• good information on E, confounders, especially if prospective
• can study the effect of an E on multiple outcomes
• less vunerable to bias

Question 18

what are the limitations of cohort studies?

Answer 18

• many need a large number of subjects to be followed for a long time
• expsenive and time consuming
• not good for rare disease or those with long latecny if prospective
• LTF can undermine validity

Question 19

what is the goal of a cohort?

Answer 19

• obtain a complete follow up info on all subjects regardless of exposure status
• very time consuming porcess

Question 20

what is LTF?

Answer 20

• loss to follow up - raises doubts about the validity of a study
• high LTF = bias
• decrease in sample size
• no magical number for it to be okay but preferred to be >80%

Question 21

what is a prospective cohort study

Answer 21

• participants are grouped on teh basis of past or current exposure and followed into the future yo observe the outcome of interests
• looking forward in time

Question 22

prospective cohort study limitations and strengths

Answer 22

• limitations
  expensive and time consuming
  not efficient for diseases with long latent period
• strengths
  better exposure and confounder data (real time collection of data)
  less venerable to bias (since outcome has not happened yet)

Question 23

what is a retrospective study?

Answer 23

• exposure and outcome has already occurred when the study began; studies past E and outcomes
• looking back on time and starting where no one has a disease in the beginning

Question 24

what are the strengths and limitations of a retrospective study?

Answer 24

• strengths
  cheaper and faster
  efficient with disease with long latent period
• limitation
  exposure and confounder data may be inadequate
  more venerable to bias

Question 25

what is ambidirectional?

Answer 25

• both componets; goes back in time to assess the exposure of the pop (everyone will disease free)
• followed through current and future

Question 26

Describe in one sentence what the key feature is that differentiates a Randomized Clinical Trial (otherwise
known as an Experimental Study) from a Cohort Study

Answer 26

Cohort studies are the observational equivalent to experimental studies with the exception that the researcher
does not allocate “exposure” (does not randomize exposure status). In experimental studies, the research
assigns exposure to the study participants.

Question 27

Prospective cohort
study group?
Absolute measures of comparison?
Relative measures of comparison?
Temporal relationship between E and D?
Multiple associations?
Time required for study?

Answer 27

• exposed persons
• unexposed persons
• RD/AR PRD/PAR APe/AR% APt/PAR%
• relative risks and Disease OR
• easy to establish
• possible to study association of an E with multiple diseases
• generally long because of the need to follow the subjects

Question 28

prospective study
- the cost of the study?
- estimate differences between groups studied?
- Is population size required?
- best when (E is rare or not/D is rare or not ) ?
- problems?
- potential bias?

Answer 28

• expensive
• medium to large
• relatively large
• exposure is rare and disease is frequent among exposed
• selection fo nonexposed comparison group is often difficult and changes over time in criteria and methods
• assessment of outcome