Lectures #22-23

Question 1

Observational, analytical studies allowing researcher to be a passive observer of natural events occurring in naturally-exposed and unexposed (comparison) groups

Answer 1

Cohort Studies

Question 2

How is group-allocation based in a cohort study

Answer 2

on exposure-status or group membership (something in common)

Question 3

When are Cohort studies most useful

Answer 3

When studying a rare exposure

Question 4

Incidence studies/follow-up studies/longitudinal studies are different names for

Answer 4

Cohort Studies

Question 5

Cohort studies commonly generate the ______ as measure of association

Answer 5

Risk Ration (RR)

Question 6

What is the difference in measure of association generated by cohort and Case-control studies

Answer 6

Cohort- Risk Ratio (RR)

Case-control- Odds Ration (OR)

Question 7

At the start of a cohort study we know what part of the 2x2 table

Answer 7

Row totals

Question 8

What are the reasons to select a cohort study

Answer 8

• unable to “randomize”: this can be do to unethical/illegal/otherwise not feasible
• The exposure of interest is rare in occurrence and little is known about its associations/outcomes
• more interested in incidence rates/predictors of or risks for outcome of interest (more than effects of interventions)

Question 9

Cohort studies can be conducted in what fashion (s)

Answer 9

Prospective, Retrospective (historical), or Ambidirectional fasion

Question 10

In what kind of cohort study have both the exposure and outcome of interest already occurred at the start of the study

Answer 10

Restrospective (historical) cohort studies (note that exposure still has to occur before outcome of interest and group allocation is based on exposure status, not disease status)

Question 11

Cohort study that uses retrospective design to assess past differences but adds all data collected on additional outcomes prospectively from start of study

Answer 11

Ambidrectional cohort study

Question 12

What is a Birth cohort

Answer 12

individuals assembled based on being born in a geographic region in a given time period

Question 13

What is an inception cohort

Answer 13

individuals assembled at a given point based on some common factor: such as where people live or where they work, or something they have in common

Question 14

When is a inception cohort useful

Answer 14

for single-group non-comparisons for incidence rate determination

Question 15

What is an exposure cohort

Answer 15

individuals assembled based on some common exposure

- most connected to environmental or other one-time events

Question 16

What is a fixed cohort

Answer 16

A cohort (derived form an irrevocable event) which can’t gain members but can have loss-to-follow-ups

Question 17

What is a closed cohort

Answer 17

A fixed cohort with no loss-to-follow-ups

Question 18

What is an open (or Dynamic) Cohort

Answer 18

A cohort with new additions and some loss to follow ups

Question 19

How to select exposed study population for cohort study

Answer 19

allocate subjects based on pre-defined criteria of “exposure”

Question 20

How to select unexposed for cohort study

Answer 20

make the groups as close as possible (coming from the same cohort/population (yet not exposed))

Question 21

If exposure truly has no effect than

Answer 21

the risk will be exactly the same for both groups and the risk ration (RR) will be 1.0

Question 22

What are the three sources the unexposed group in a cohort study can come from

Answer 22

Internal (best, if feasible)
- same cohort but are unexposed
- if there are only levels of exposure, you may have to
use the lowest exposure group as comparison
General Population
- second choice
- used when internal group is not possible (e.g. everyone is exposed, or the exposure subjects were drawn from the general population)
Comparison Cohort
- Least acceptable
- Simply attempt to match groups as close as possible on numerous personal characteristics ( can’t control for other potentially harmful exposures in comparison cohort; also causing disease

Question 23

Strengths of a Cohort study

Answer 23

• Good for assessing multiple outcomes of one exposure
• Useful when exposures are rare
• Useful in calculating risk and RR’s
• Less expensive than interventional studies (in general)
• good when ethical issues limit use of interventional studies
• Good for long induction/latent periods (retrospective studies)
• Able to represent “temporality” (Prospective)

Question 24

What are the advantages to a prospective cohort study

Answer 24

• Can obtain a greater amount of study-important information from patients
  
  • more control over specific data collection processes
• Follow-up/Tracking of patients may be easier
• Better at giving answer to “Temporality”
• May look at multiple outcomes form a (supposed) single exposure
• Can calculate incidence and Incidence rates

Question 25

What are the disadvantages of a prospective Cohort

Answer 25

Time, Expense and lost-to-follow up
Not efficient for rare diseases
Not suited for long induction/Latency conditions
Exposure (or its “amount”) may change over time

Question 26

What kind of observational study is used for rare diseases

Answer 26

Case-Control study

Question 27

Loss to follow up (LTFU’s) increases what type of error

Answer 27

Type 2 error

Question 28

Do authors’s have to list the LTFU’s by group (exposed/unexposed)

Answer 28

yes

Question 29

What are the advantages of a retrospective cohort study

Answer 29

Best for long induction/latency conditions
able to study rare exposures
useful if the data already exists
save time and money compared to prospective studies

Question 30

What are the disadvantages of retrospective cohort studies

Answer 30

• Requires access to charts, databases, employment records
• “information” may not factor in or control for other exposures to harmful elements
• patients may not be available for interview if contact necessary for missing or incomplete data
• exposure (or its “amount”) may have changed over time

Question 31

issues affecting outcome occurrence in groups of cohort studies

Answer 31

• Levels of exposure
• Induction period
• Latency Period

Question 32

What are the two key biases with cohort studies

Answer 32

• Healthy-worker effect

- Selection bias