7: Cohort Study Design Flashcards
Temporality
refers to the timing of information about cause and effect. demonstrating temporality is a difficulty of most observational studies
Cross-sectional and case-control study designs are based on exposure and disease information that is collected at the same time. Advantage? Disadvantage?
Advantage: efficient for generating and testing hypotheses
Disadvantage: leads to challenges regarding interpretation of results
Limitations of Study Designs of Cross-Sectional, Ecologic, and Case-Control (2)
- No actual lapse of time between measurement of exposure and disease
- not well suited for uncommon exposures
cohors
referred to one of ten divisions of an ancient Roman legion
cohort
a population group, or subset thereof, that is followed over a period of time
cohort group members experience a common exposure:
- associated with a specific setting
2. share non-specific exposure associated with a general classification (birth cohort)
birth cohort
being born in the same year
Cohort Effect
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)
Cohort Analysis
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
Wade Hampton Frost
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.
Life Table Methods
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.
Two Life Table Methods
- Cohort Life Table
2. Period Life Table
Cohort Life Table
shows the mortality experience of all persons born during a particular year
Period Life Table
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
Years of Potential Life Lost (YPLL)
computed for each individual in a population by subtraction that person’s life span from the average life expectancy of the population
Disability-Adjusted Life Years (DALYs)
adds the time a person has a disability to the time lost to early death
Survival Curves
method for portraying survival times
required info in order to construct a survival curve (3)
- time of entry into the study
- time of death or other outcome
- status of patient at time of outcome (dead or censored (lost to follow-up))
inverse of survival curve
lethality curve. useful for insecticides, etc.
Cohort Study
- 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
Cohort studies differ according to:
sampling strategy used. population-based vs. exposure-based
Population-Based Cohort Study
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.
Framingham Study
Population-Based. ongoing study of CHD. used random sample of town from targeted age range (not entire town population)
Tecumseh Study
Population-Based. total community cohort. examined contribution of environmental and constitutional factors to maintenance of health and origins of illness
Levels of exposure
- Dichotomous variable (exposed vs. nonexposed)
- Continuous variable (blood pressure measurement)
Exposure-Based Cohort Study
made up of subjects with a common exposure. GREAT FOR RARE EXPOSURE! certain groups may have higher exposures than general population to specific hazards
Comparison (Non-Exposed Group)
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)
Outcome Measures
- Discrete Events (single events and multiple occurrences)
- Levels of Disease Markers
- Changes in Disease Markers (rate of change)
Prospective Cohort Study
determination of exposure levels (present) and follow-up for occurrence of disease (future)
EXAMPLE: The National Children’s Study
Prospective Cohort
Prospective Cohort Advantages
- 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
Prospective Cohort Disadvantages
expensive and labor intensive, have to wait for cases to accrue
Retrospective Cohort Study
make use of historical data to determine exposure level (past) and collect occurrence of disease (present)
How is retrospective cohort different than a case-control
dont know who will get disease, case-control does
Retrospective Cohort Advantages
- significant amount follow-up accrued in short period of time
- extensive amount of exposure data collected and available at minimal cost
Retrospective Cohort Disadvantages
cant recall exposure, or cant access exposure data
Historical Prospective Cohort Study
makes use of both retrospective features ( to determine baseline exposure, past and present) and prospective features (to determine disease incidence, future).
Ambispective Cohort Study
another term for Historical Prospective
Practical Considerations Regarding Cohort Studies (5)
- Availability of exposure data
- size and cost of the cohort used
- Data collection and data management
- Follow-up issues
- Sufficiency of scientific justification
- Availability of Exposure Data
High quality historical exposure data are absolutely essential for retrospective cohort studies
- Size and Cost of the Cohort Used
The larger the size of the cohort, the greater the opportunity to obtain findings in a timely manner. Resource constraints typically influence design decisions
- Data Collection and Data Management
Explicit protocols for quality control. organizational and administrative burdens increased with multiple levels of data collection
- Follow-up Issues
cohorts only effective with follow-up. Active vs. Passive follow-up
Active Follow-up
investigator obtains data through direct contact with cohort
Passive Follow-up
record linkage between databases. data collected and maintained by organizations outside investigative team.
- Sufficiency of Scientific Justification
need considerable scientific rationale for cohort study.
cohort studies are the only observational study design that…?
permits examination of multiple outcomes
Relative Risk
direct measure of association between exposure and outcome. [A/A+B]/[C/C+D] Exposure > Result
Attributable Risk
“Risk Difference” [A/A+B]-[C/C+D] great for occupational health
RR=1
risk (rate) of disease among exposed is no different from risk of disease in non-exposed
RR>or =2
risk is twice as high
RR<or=0.5
protective factor, exposure associated with half the risk of disease
Nested Case-Control Studies
type of case-control study in which cases and controls are drawn from the population in a cohort study
Nested Case-Control Advantages
- degree of control over confounding factors
- reduce cost because exposure information is collected from subset of cohort only
Advantages of Cohort Studies
- direct determination of RISK
- TIME SEQUENCING of exposure and outcome
- can study MULTIPLE OUTCOMES
- can study RARE EXPOSURES
Limitations of Cohort Studies
- take a LONG TIME
- COSTLY
- SUBJECTS LOST to follow-up
