Chort Studies

Question 1

What is the purpose of a cohort study?

Answer 1

Studying rare exposure. commonly generates the risk of disease/outcome for each and then a Risk Ratio/Relative RIsk as a measure of Assoc.

May not always describe how groups are allocated

Question 2

What are the reasons to select a cohort study?

Answer 2

unable to force group allocation (randomize)

Unethical, not feasible

Limited resources - Time/Money/Subjects

Exposure of interest is rare in occurance and little is known about its associations and outcomes

More interested in the actual incidence rates or risks for outcome of interest more than effects of interventions

Question 3

What are the different routes of a Cohort study?

Answer 3

Prospective, retrospective, ambiderectional

Group assignment based on EXPOSURE STATUS or GROUP MEMBERSHIP and it MEASURES OUTCOME

does NOT prove causation !!!!

Question 4

What is a Prospective Cohort Study?

Answer 4

EXposure group is selected on past or current exposure and both groups are followed INTO THE FUTURE to assess outcomes of interest and compare them

Question 5

What is a retrospective cohort study?

Answer 5

At the start of the study both exposure and outcome have already occured but groups are still allocated based on past history of exposure

Exposure still has to occur before outcome of interested and group allocation is based on exposure status NOT DIEASE STATUS

Question 6

What is an ambidirectional Cohort study?

Answer 6

Uses retrospective design to assess past differences (up to present) but also adds future data collected on additonal outcomes prospectively from start of study

looking for otcomes in past, and known in the present but also in the future

Question 7

What is a cohort?

Answer 7

refers to a group with something in common

Ex: everyone born in kansas city in 2014

Question 8

What is an inception cohort?

Answer 8

Individuals assembled at a given point based on a some common factor

first responders on 9/11

Question 9

What is a exposure cohort?

Answer 9

Individuals assembled based on some common exposure

Question 10

What are the three different types of cohorts sizes ?

Answer 10

Fixed - no more additons but they lose numbers to follow ups

Closed - Fixed cohort but they also can not lose any numbers because there is an extremely short study time

Open (or Dynamic) cohort - New additions and some loss to follow ups

Question 11

How are group allocations accomplished?

Answer 11

• MAke groups as closed as possible
• selection of exposed study population
  
  • Allocation of subjects based on predefined criteria of exposures is for exposed studied population
• Selecting Unexposed
  
  • should come from the same populations not yet exposed
  • should come from 3 sources
    
    • Internal
      
      • patients from same cohort yet who are unexposed (most similair )
      • levels of exposure may result in using lowest esposure group
    • general populations
      
      • Used as a second choice
    • comparison cohort
      
      1. least acceptible
      • attempting to match groups as close as possible on numerous personal charcteristics and cant control for other potentially harmful exposures in comparison cohort )

Question 12

What are strengths of a cohort study?

Answer 12

• good for assessing mutliple outcomes of one exposures
• when exposures are rare
• calc risks and RR
• less expensive than interventional
• Good ethical issues limit use of interventional
• Retrospective - good for long induction/latent peroids
• Propspective - represents temporality

Question 13

What weaknesses of Cohort studies

Answer 13

CANT demostrate Causation

hard to control for other exposures if more than one plausible for being associated with an outcome

retrospective cant control for other exposures or potential changes in amount of study exposure during study frame

Prospective not good for long inducton/latent peroids

impacted by unassessed confounders (more so with retrospective)

can be impacted by various biases most importantly selection and recall biases (retro)

Limited by avalible data (Retro)

Question 14

what are advantages of Prospective cohort Studies

Answer 14

• Can obtain greater amount of study-important information from patients
• follow up/tracking of patients may be easier
• Better at giving answers to temporality
• may look at multiple outcomes from a supposed single exposure
• can calculate incidence and incidence rates

Question 15

What are the disadvantages of prospective cohort studies?

Answer 15

TIme, expense, and lost to follow ups

Not efficient for rare dieases

not suited for long induction/latency conditions

exposure or its amount may change over time

Question 16

What assocatied with LTFU or lost to Follow ups?

Answer 16

• Possible with prospective cohorts
• lowers sample size (power)
• Increased risk of type 2 error
• Loss of study participation (follow up) may not be equal b/w groups

Question 17

What are advantages of retrospective?

Answer 17

Best for long inductions/Latency conditions

Able to study rare exposures

useful if the data already exists

saves time and money compared to prospective studies

Question 18

What are the disadvantages of retrospective cohort studies ?

Answer 18

• requires access to data base records
• information may not factor or control other exposures to elements
• patients may be LTFU for missing or unfinished data
• expoures may have changed over time

Question 19

What is matching?

Answer 19

A way to make groups as equal as possible on known/potential confounders

can match on 1:1 or even higher at 1:5 ratio (exposed to unexposed )

Question 20

What are key biases with cohort studies ?

Answer 20

healthy worker effect

selection bias