Module 8

True or False? A retrospective cohort study is more efficient than a prospective cohort study for studying diseases with long latent and induction periods.

True

True or False? General cohort studies are good for studying common exposures, whereas special cohorts are good for studying rare ones.

True

True or False? Observational studies are considered "natural" experiments because the investigator acts as a disinterested observer, merely letting nature take its course.

True

True or False? The timing of prospective cohort studies is similar to that of experimental studies?

True

What is the ideal but unattainable comparison group in a cohort study?

a group of exactly the same individuals in the exposed group had they not been exposed (AKA the counterfactual ideal because it is impossible for the same person to be exposed and unexposed at the same time)

What techniques used in experimental studies are also applicable for cohort studies? a. Blinding (Masking) b. Placebo c. Randomization d. Run-in period e. Follow-up

a. Blinding (Masking) e. Follow-up

Which type of comparison group is commonly used in occupational studies of mortality because these data are quite accessible to researchers? a. General population b. Internal comparison group c. External comparison group d. Comparison cohort

a. General population

Which of the following is the appropriate measure of disease frequency in a fixed cohort when the population experiences losses to follow-up? a. Incidence rates b. Prevalence rates c. Mortality rates d. Cumulative incidence rates

a. Incidence rates

Which of the following techniques that are commonly used in experimental studies can also be applied in cohort studies? a. Masking b. Placebo c. Randomization d. Run-in period e. Intent-to-treat analysis

a. Masking

State whether or not a cohort study is best suited for each of the following scenarios: a. When little is known about a rare exposure b. When little is known about a rare disease c. When the study population will be difficult to follow d. When you want to learn about multiple effects of an exposure

a. Yes b. No c. No d. Yes

study that has both prospective and retrospective components

ambidirectional cohort study

Which of the following is often selected from professional groups such as doctors and nurses, volunteers, or residents in well-defined geographic areas to facilitate follow-up and accurate ascertainment of the outcomes under study? a. Open cohort b. Fixed cohort c. Special cohort d. General cohort

d. General cohort

defined by an irrevocable event so it does not gain any new members

fixed cohort

defined by a changeable characteristic (i.e. residence in a specific city); members are in a dynamic population, so they may come and go depending on whether or not they have the specific characteristic

open or dynamic cohort

How is person-time calculated within the context of a cohort study?

person-time is calculated for each individual in the study, beginning when the follow-up period of the study begins and ending when/if the individual develops the outcome, dies, is lost to follow-up, or the follow-up period ends for the study.

study that looks forward in time and examines future outcomes in relation to past or current exposures

prospective cohort study

study that looks back in time and examines exposures and outcomes that have already occurred by the time the investigator begins the study

retrospective cohort study

What is the main similarity and main difference between experiments and cohort studies?

similarity: both compare exposure groups and see incidence of outcomes difference: ability/inability to assign treatment (experiments do it and cohort does not)

special study in which the mortality experience of an exposed group is compared with that of the general population (commonly used in occupational settings)

standardized mortality ratio (SMR) study

strength and weakness of the internal comparison group

strength: comes the closest to the counterfactual ideal because it comes from the same source population as the exposed group so it is most comparable weakness: often difficult to identify

strength and weakness of the comparison cohort

strength: may be comparable to the exposed group weakness: results are difficult to interpret because this type of cohort usually has other possibly dangerous exposures

strength and weakness of the general population

strength: stable and easy to obtain weakness: may be difficult to compare to the exposed group and there may be a lack of information on confounding factors

True or False? Losses to follow-up can be a problem in a cohort study but not in an experimental study.

False

True or False? Special cohort studies are the most sensible design for examining many exposures in relation to a single disease.

False

True or False? Investigators in a retrospective cohort study can usually obtain more detailed information on exposures and other key variables because they have more control of the data collection process and can gather information directly from the participants.

False

Among the following studies, which comparison group comes closest to the counterfactual ideal? a. A study that compares male breast cancer rates among U.S. Marines who were stationed at Camp Lejeune, North Carolina, and were exposed to solvents in the local drinking water with breast cancer rates among men from the general U.S. population. b. A study that compares male breast cancer rates among U.S. Marines who were stationed at Camp Lejeune, North Carolina, and were exposed to solvents in the local drinking water with U.S. Marines who were stationed at Camp Lejeune, North Carolina, but were not exposed to solvents in the local drinking water. c. A study that compares male breast cancer rates among U.S. Marines who were stationed at Camp Lejeune, North Carolina, and were exposed to solvents in the local drinking water with U.S. Marines who were stationed at Camp Pendleton, California, and were not exposed to solvents in the local drinking water.

b. A study that compares male breast cancer rates among U.S. Marines who were stationed at Camp Lejeune, North Carolina, and were exposed to solvents in the local drinking water with U.S. Marines who were stationed at Camp Lejeune, North Carolina, but were not exposed to solvents in the local drinking water.

Exposures in which type of cohort do not change over time and are followed from a defined starting point to a defined ending point but may suffer from losses to follow-up? a. Open cohort b. Fixed cohort c. Closed cohort d. Dynamic cohort

b. Fixed cohort

Epidemiologists have found that which type of comparison group is the best type of comparison group to use in a cohort study? a. General population b. Internal comparison group c. External comparison group d. Comparison cohort

b. Internal comparison group

Who can be blinded or masked in a cohort study? a. Participants b. Investigators who do the follow-up c. Investigators who analyze results

b. Investigators who do the follow-up c. Investigators who analyze results

If lost individuals are more or less likely to be exposed than those successfully traced, the study results may suffer from which of the following? a. Confounding b. Selection bias c. Counterfactual ideal d. Information bias

b. Selection bias

Why is it important to minimize losses to follow-up?

because they decrease the number of individuals who can be included in the analysis, reducing the statistical power of the study. plus, if the people lost to follow-up have different rates of disease than those that are not lost, the study results may be biased

Which of the following is assembled to study the health effects of rare exposures, such as uncommon occupational chemicals? a. Open cohort b. Fixed cohort c. Special cohort d. General cohort

c. Special cohort