PHP 405 Final

describe the strengths and weaknesses of the three types of comparison groups used in cohort studies. which one comes closes to the counterfactual ideal?

-Internal comparison: comes closest to the counterfactual ideal because it comes from the same source population as the exposed group and so is most comparable. However, they are often hard to identify -General population: it is stable and easy to obtain. includes a lack of comparability to the exposed group and lack of information on confounders. -comparison cohort: least preferable option. although it may be comparable to the exposed group, results from such a study are hard to interpret because the comparison cohort often has other exposures.

state the main difference between differential and non differential misclassification, and state which directions each type of error can bias the study results

-Nondifferential missclassification, inaccuracies that occur on one axis (exposure or disease) are independent from the other axis. of dichotomous variables biases the results towards the null -differential missclassification, inaccuracies that occur on one axis (exposure or disease) are dependent on the other axis. can bias the results either towards or away from the null

describe two methods for controlling confounding during the analysis, and give one advantage and one disadvantage for each method

-Stratification: process of evaluating the association within homogeneous categories of a confounder. straightforward and easy to carry out. it cannot control for numerous variables because a large number of strata are generated relative to the number of study subjects. -Multivariate analysis: a method for controlling confounding by constructing a mathematical model that describes the association between the exposure, the outcome, and confounders. it can control for may confounders simultaneously. One can not longer view the raw data

B. odds ratio

-The odds of being a case among the exposed compared with the odds of being a case among the nonexposed -the odds of being exposed among the cases compared with the odds of being exposed among the controls

E. age specific rate and age adjusted rate

-age specific rate is a rate that applies only to a particular age group -age adjusted rate is a summary rate that accounts for the age differences between two populations

A. prevalence and incidence

-prevalence quantifies existing cases -incidence quantifies new cases

describe three methods for controlling confounding in the study design, and give one advantage and one disadvantage for each method.

-randomization: act of assigning or ordering using a random process. it controls both known and unknown confounders, if sample size is large enough. it can only be used in experimental studies -Matching: process of making the distribution of confounders identical in the compared groups while selecting the study subjects. good for controlling for confounding by complex nominal variables and for controlling confounding in small studies. It is difficult and expensive finding appropriate matches -restriction: means that the investigator limits admission into a study to individuals who fall within a specific category or categories of a confounder. simple and relatively low expense. but difficult in identifying a sufficient number of subjects and limiting generalizability of the study

describe the situations in which it is desirable to conduct a case-control study

-when the exposure data is difficult or expensive to obtain, when the disease is rare, when the disease has a long induction and latent period, when little is known about the disease, and when the underlying population is dynamic.

Recall bias is most likely to occur in: A. case-control studies B. prospective cohort studies C. experimental D. all of the above E. none of the above

A

A. cause of disease

A cause of a disease is an event, condition, or characteristic that preceded the disease and without which the disease either would not have occurred or would have occurred later

which of the following techniques that are commonly used in experimental studies can also be applied to cohort studies? A. Blinding B. Placebo C. randomization D. Run-in period

A. Yes B. No C. No D. No

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 up D. when you want to learn about multiple effects of an exposure

A. Yes B. No C. No D. Yes

Epidemiologists plan the appropriate size for a study by: A. using judgement, experience, and intuition B. performing sample size calculations C. both A and B D. Neither A nor B

C

Selection bias is most likely to occur in: A. case-control studies B. retrospective cohort studies C. experimental studies D. both retrospective and case-control E. both retrospective and experimental

D

how do you determine if a variable confounds an association?

Epidemiologists usually compare the crude/confounded measure of association with the adjusted measure of association. If there is an appreciable difference between the two, confounding is considered present.

it is possible to obtain a valid estimate of disease prevalence from a typical case-control study

False

A. Confounding

Is a mixing of effects between an exposure, an outcome, and a third extraneous variable that is termed the confounder. Confounding distorts the crude relationship between an exposure and outcome because of the relationships between the confounder and the exposure, and the confounder and the disease

why is it important to minimize loss to follow-up?

Loss to follow-up decreases the number of individuals who can be included in the analysis and so reduces the statistical power of the study. also, if those who are lost have different rates of disease than those who remain, the study results may be biased

A. Interviewer bias

Mask interviewers to the study hypothesis and to the disease or exposure status of the study subjects, and carefully design the interview instrument

why are P values considered confounded statistics?

P-values are affected both by the magnitude of the association and the study size, thus, when results are summarized only by p-values, it is impossible to determine if a p value is small because the measure of association is strong or because the sample size is large. it is also impossible to determine if a P value is large because the association is weak or the sample size is small

Risk Difference associations (RD)

Rate or risk in exposed (Rexp)-rate or risk in unexposed (Runexp) RD= 1 (excess risk of disease) RD= 2 (excess risk of 2) RD= .05 (weak association) RD= 0 (no association)

A. TROHOC and TROHOC fallacy

TROHOC is the word cohort spelled backwards. Some epidemiologists use TROHOC as a disparaging term for case-control studies because they believe that case-control studies are inferior to cohort studies. TROHOC fallacy means that it is incorrect to consider the logic of a case-control study backwards, because the key comparison is identical to that of a cohort study.

State the main similarity and main difference between cohort and experimental studies?

The main similarity is that both compare two or more exposure groups, which are followed to monitor outcome rates. The main difference is that the investigators allocate the exposure in experimental studies, and the participants choose their exposure in cohort studies

what are the key characteristics of a confounding variable?

a confounder is associated with the exposure in the source population that produced the cases and an independent cause or predictor of the outcome under study. the latter means that it is associated with the disease among both exposed and unexposed individuals. in addition, a confounder cannot be an intermediate step in the causal pathway between the exposure and disease

state the main advantages and disadvantages of case-control studies

advantages: case-control studies take less time and money to conduct than cohort and experimental studies, they are well suited for studying rare diseases and diseases with long induction and latent periods, and they can provide information on a large number of possible risk factors disadvantages: the possibility of bias is increased, and it may be difficult to establish a correct temporal relationship between the exposure and disease because the data are retrospective.

describe one advantage and one disadvantage of using population controls in a case-control study

advantages: they usually come from the same source population as the cases and so they are likely to be comparable. disadvantages: they are time-consuming and expensive to identify, they are usually not as cooperative as hospital controls, and their recall of prior exposures may not be as accurate as that of cases

D. risk difference and population risk difference

both provided information on the absolute effect of the exposure or the excess risk of disease -risk difference gives the number of cases of disease among the exposed that may be attributable to the exposure -population risk difference gives the number of cases of disease in the total population that may be attributable to the exposure

give one reason why many epidemiologists prefer to use confidence intervals instead of P values to assess the role of random errer

confidence intervals are not confounded statistics like P values. they do a better job separating the influence of the sample size from the influence of the strength of the association. this is because the width of the interval is influenced mainly by the sample size, and the general position of the interval reflects the magnitude of the assocation

what measure? the lifetime risk of breast cancer

cumulative incidence

what measure? the percentage of freshman girls who become pregnant over the course of their high school years

cumulative incidence

C. selection bias

ensure that selection of cases and controls is independent of exposure (in case-control study)and that selection of exposed and unexposed groups is independent of outcome (in retrospective cohort) and obtain high follow-up and participation rates (all studies)

B. Bias is introduced primarily during the analysis stage of a study

false

B. Intermediate variables in a causal pathway are special types of confounders

false

B. reducing random error also reduces errors from bias and confounding

false

C. a causal relationship between an exposure and disease cannot be established unless all of Hill's guidelines are met

false

D. epidemiologists can tell if confounding is present by examining the strength of the crude measure of association

false

D. using an inaccurate case definition increases the likelihood of non-differential misclassification of the disease

false

E. a statistically significant finding always has public health significance

false

E. experimental studies always have less confounding than observational studies

false

E. including a large sample size reduces self-selection bias

false

F. Exposure to HIV is a sufficient cause of AIDS

false

F. poor recall and recall bias are synonymous terms for the same concept

false

G. exposure to cigarette smoke is a necessary cause of lung cancer

false

H. exposure to cigarette smoke is a sufficient cause of lung cancer

false

a case-control study is the most efficient design for studying the health effects of rare exposures, while a cohort study is the most efficient design for studying the risk factors for rare diseases

false

briefly define each of the following terms:

false

case identification is generally more difficult than control identification in case-control studies

false

cohort studies are the most sensible design for examining many exposures in relation to a single disease

false

loss to follow-up can be a problem in a cohort study but not an experimental study

false

the control group in a case-control study should never include individuals who have the cases's disease

false

the odds of illness are mathematically equivalent to the risk of illness

false

the purpose of a control group in a case-control study is to provide information on the disease distribution in the source population that produced the cases

false

what measure? the number of live born babies who die of SIDS during the first year of life per 100,000 baby years of follow up

incidence rate

C. Necessary cause

is a component cause that is a member of every sufficient cause

C. Statistical inference

is a method for generalizing results from a sample to a parent population

C. case-crossover study

is a new variant of the case-control study that is used to study the acute effects of transient exposures. Here, cases serve as their own controls, and the exposure frequency during a hazard period is compared with that during a control period.

B. Sufficient cause

is a set of conditions without any one of which the disease would not have occurred

C. control selection bias

is a type of selection bias that occurs in case-control studies when the controls do not accurately represent the exposure distribution in the source population that produced the cases. it occurs when different criteria were used to to select cases and controls

A. Chance

is an uncontrollable force that seems to have no assignable or predictable cause

B. precision

is the lack of random error. it is defined either as the state or quality of being exact or the ability of a measurement to be consistently reproduced

B. Recall Bias

mask study subjects to the study hypothesis, use diseased controls if conducting a case-control study, and carefully design an interview instrument

B. residual confounding

means that an association remains confounded even after some confounders have been controlled. It arises from lack of information on all confounding variables, classifying confounders in overly broad categories, or mismeasuring confounders.

B. Healthy worker effect

occurs in occupational studies when disease and death rates in a working population are compared with those among the general population.

A. Recall Bias

occurs when the level of accuracy differs between the compared groups. -in case-control study when cases remember or report their exposures differently from controls -in cohort study when individuals who are exposed remember or report subsequent illnesses differently than those who are unexposed

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

person-time is accrued for each individual in a cohort study. it begins when the follow-up period of the study begins. it ends when one of the following occurs: the individual develops the outcome under study, dies, is lost, or the follow-up period for the study ends

C. positive and negative confounding

positive confounding means that true crude association is exaggerated. away from the null negative confounding means that the true crude association is underestimated. toward the null

what measure? the percentage of infants weighing less than 2,500 grams at birth?

prevalence

what measure? the percentage of senior boys who are fathers at the time of graduation

prevalence

B. incidence rate and cumulative incidence

similarity: both quantify the number of new cases of disease that develop in a population at risk during a specified period of time difference: incidence rate is a true rate that directly integrates person-time of observation into the denominator and cumulative incidence is a proportion whose denominator is the population at risk at the start of the observation period

what is the main limitation of significance testing?

the chief limitation is the use of purely arbitrary cutoff for deciding whether or not to reject the null hypothesis

What is the main assumption involved in hypothesis testing and the calculation of P values?

the main assumption is that the null hypothesis is true

state the probability distributions that are commonly used in epidemiological research and describe the settings in which they are used

the normal distribution is used for continuous variables, and the binomial and Poisson distributions are used for discrete variables with two mutually exclusive outcomes. In addition, the poisson distribution is usually reserved for rare events

Describe the temporal relationship between the induction period and latent period

the overall induction period begins with the action of the first causal component and ends with the action of the last causal component and the simultaneous biological onset of disease. The latent period follows the induction period and so begins with the biological onset of disease and ends with the disease diagnosis

C. incidence rate ratio and incidence rate difference

they are both ways to compare measures of disease frequency in order to assess the impact of an exposure on a disease -the ratio measure gives information on the strength of the relationship between an exposure and disease -the difference measure describes the excess number of cases of disease that are associated with the exposure

A. Hill's guidelines of temporality is more easily established in a prospective than retrospective study

true

A. Unlike bias and confounding, random errors are unsystematic

true

A. a study must be valid before its results can be generalized

true

A. all high-quality epidemiological studies include techniques for controlling confounding

true

B. Strong associations are more likely to be causal than weak ones because they are less likely to be due to alternative explanations

true

C. The counterfactual ideal is used to guide the selection of a comparison group in order to minimize confounding

true

C. bias can pull an estimate of association either toward the null or away from the null

true

C. precise exposure data can be achieved by repeating the exposure measurements

true

D. Hill's guidelines of specificity means that an exposure can cause only one disease

true

D. Increasing the sample size decreases the change of selecting an unrepresentative sample

true

E. exposure to HIV is a necessary cause of AIDS

true

a retrospective cohort study is more efficient than a prospective cohort study for studying diseases with a long latent and induction period

true

the ideal comparison group for a cohort study would consist of exactly the same individuals in the exposed group had they not be exposed

true

D. misclassification

use the most accurate source of information, and use sensitive and specific criteria to define the exposure and disease