7: Cohort Study Design

Question 1

Temporality

Answer 1

refers to the timing of information about cause and effect. demonstrating temporality is a difficulty of most observational studies

Question 2

Cross-sectional and case-control study designs are based on exposure and disease information that is collected at the same time. Advantage? Disadvantage?

Answer 2

Advantage: efficient for generating and testing hypotheses
Disadvantage: leads to challenges regarding interpretation of results

Question 3

Limitations of Study Designs of Cross-Sectional, Ecologic, and Case-Control (2)

Answer 3

1. No actual lapse of time between measurement of exposure and disease
2. not well suited for uncommon exposures

Question 4

cohors

Answer 4

referred to one of ten divisions of an ancient Roman legion

Question 5

cohort

Answer 5

a population group, or subset thereof, that is followed over a period of time

Question 6

cohort group members experience a common exposure:

Answer 6

1. associated with a specific setting

2. share non-specific exposure associated with a general classification (birth cohort)

Question 7

birth cohort

Answer 7

being born in the same year

Question 8

Cohort Effect

Answer 8

the influence of membership in a particular cohort (less than 5% population smoked in early 1900s > free cigs for WW1 troops > shift in distribution of age of onset of lung cancer)

Question 9

Cohort Analysis

Answer 9

the tabulation and analysis of morbidity or mortality rates

• with relationship to ages of a specific cohort identified
• with respect to a particular period of time
• followed as they pass through different ages during part or all of their life span

Question 10

Wade Hampton Frost

Answer 10

Arranged tuberculosis mortality rates in a table with age on one axis and year of death on the other. Drew substantial attention to the cohort analysis method.

Question 11

Life Table Methods

Answer 11

Give estimates for survival during time intervals and present the cumulative survival probability at the end of the interval. Can be constructed to portray the survival times of patients in clinical trials.

Question 12

Two Life Table Methods

Answer 12

1. Cohort Life Table

2. Period Life Table

Question 13

Cohort Life Table

Answer 13

shows the mortality experience of all persons born during a particular year

Question 14

Period Life Table

Answer 14

enables us to project the future life expectancy of persons born during the year as well as the remaining life expectancy of persons who have attained a certain age

Question 15

Years of Potential Life Lost (YPLL)

Answer 15

computed for each individual in a population by subtraction that person’s life span from the average life expectancy of the population

Question 16

Disability-Adjusted Life Years (DALYs)

Answer 16

adds the time a person has a disability to the time lost to early death

Question 17

Survival Curves

Answer 17

method for portraying survival times

Question 18

required info in order to construct a survival curve (3)

Answer 18

1. time of entry into the study
2. time of death or other outcome
3. status of patient at time of outcome (dead or censored (lost to follow-up))

Question 19

inverse of survival curve

Answer 19

lethality curve. useful for insecticides, etc.

Question 20

Cohort Study

Answer 20

• Guided by scientific question, not availability of group for study.
• start with group of subjects who lack a positive history of outcome of interest and are at risk for the outcome.
• Includes at least two observation points: (1) exposure status and eligibility, (2) determine number of incident cases
• permit calculation of incidence rates > risk rates
• collection of primary data (initial exposure data)
• involve comparison of disease rates between exposed and non-exposed groups

Question 21

Cohort studies differ according to:

Answer 21

sampling strategy used. population-based vs. exposure-based

Question 22

Population-Based Cohort Study

Answer 22

cohort includes either an entire population or a representative sample of the population. exposures unknown until first period of observation when exposure information is collected. two or more levels of exposure. not efficient for rare exposures.

Question 23

Framingham Study

Answer 23

Population-Based. ongoing study of CHD. used random sample of town from targeted age range (not entire town population)

Question 24

Tecumseh Study

Answer 24

Population-Based. total community cohort. examined contribution of environmental and constitutional factors to maintenance of health and origins of illness

Question 25

Levels of exposure

Answer 25

- Dichotomous variable (exposed vs. nonexposed) | - Continuous variable (blood pressure measurement)

Question 26

Exposure-Based Cohort Study

Answer 26

made up of subjects with a common exposure. GREAT FOR RARE EXPOSURE! certain groups may have higher exposures than general population to specific hazards

Question 27

Comparison (Non-Exposed Group)

Answer 27

similar in demographics and geography to exposed group, but lacks the exposure. in occupational setting, several categories of exposure may exist (physician vs. billing staff)

Question 28

Outcome Measures

Answer 28

- Discrete Events (single events and multiple occurrences) - Levels of Disease Markers - Changes in Disease Markers (rate of change)

Question 29

Prospective Cohort Study

Answer 29

determination of exposure levels (present) and follow-up for occurrence of disease (future)

Question 30

EXAMPLE: The National Children's Study

Answer 30

Prospective Cohort

Question 31

Prospective Cohort Advantages

Answer 31

- enable investigator to collect data on exposures; MOST DIRECT and specific test of study hypothesis - size of cohort under greater control by investigators, recruit more individuals if needed - biological and physiological assays can be performed with decreased concern that the outcome will be affected by the under lying disease process (blood sample, lung function) - direct measures of environment can be made

Question 32

Prospective Cohort Disadvantages

Answer 32

expensive and labor intensive, have to wait for cases to accrue

Question 33

Retrospective Cohort Study

Answer 33

make use of historical data to determine exposure level (past) and collect occurrence of disease (present)

Question 34

How is retrospective cohort different than a case-control

Answer 34

dont know who will get disease, case-control does

Question 35

Retrospective Cohort Advantages

Answer 35

- significant amount follow-up accrued in short period of time - extensive amount of exposure data collected and available at minimal cost

Question 36

Retrospective Cohort Disadvantages

Answer 36

cant recall exposure, or cant access exposure data

Question 37

Historical Prospective Cohort Study

Answer 37

makes use of both retrospective features ( to determine baseline exposure, past and present) and prospective features (to determine disease incidence, future).

Question 38

Ambispective Cohort Study

Answer 38

another term for Historical Prospective

Question 39

Practical Considerations Regarding Cohort Studies (5)

Answer 39

1. Availability of exposure data 2. size and cost of the cohort used 3. Data collection and data management 4. Follow-up issues 5. Sufficiency of scientific justification

Question 40

1. Availability of Exposure Data

Answer 40

High quality historical exposure data are absolutely essential for retrospective cohort studies

Question 41

2. Size and Cost of the Cohort Used

Answer 41

The larger the size of the cohort, the greater the opportunity to obtain findings in a timely manner. Resource constraints typically influence design decisions

Question 42

3. Data Collection and Data Management

Answer 42

Explicit protocols for quality control. organizational and administrative burdens increased with multiple levels of data collection

Question 43

4. Follow-up Issues

Answer 43

cohorts only effective with follow-up. Active vs. Passive follow-up

Question 44

Active Follow-up

Answer 44

investigator obtains data through direct contact with cohort

Question 45

Passive Follow-up

Answer 45

record linkage between databases. data collected and maintained by organizations outside investigative team.

Question 46

5. Sufficiency of Scientific Justification

Answer 46

need considerable scientific rationale for cohort study.

Question 47

cohort studies are the only observational study design that...?

Answer 47

permits examination of multiple outcomes

Question 48

Relative Risk

Answer 48

direct measure of association between exposure and outcome. [A/A+B]/[C/C+D] Exposure > Result

Question 49

Attributable Risk

Answer 49

"Risk Difference" [A/A+B]-[C/C+D] great for occupational health

Question 50

RR=1

Answer 50

risk (rate) of disease among exposed is no different from risk of disease in non-exposed

Question 51

RR>or =2

Answer 51

risk is twice as high

Question 52

RR

Answer 52

protective factor, exposure associated with half the risk of disease

Question 53

Nested Case-Control Studies

Answer 53

type of case-control study in which cases and controls are drawn from the population in a cohort study

Question 54

Nested Case-Control Advantages

Answer 54

- degree of control over confounding factors | - reduce cost because exposure information is collected from subset of cohort only

Question 55

Advantages of Cohort Studies

Answer 55

- direct determination of RISK - TIME SEQUENCING of exposure and outcome - can study MULTIPLE OUTCOMES - can study RARE EXPOSURES

Question 56

Limitations of Cohort Studies

Answer 56

- take a LONG TIME - COSTLY - SUBJECTS LOST to follow-up