CPH Study Guide Epi Questions

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CPH Study Guide Epidemiology by: Sean Lando / Claudia Morales

What would be the best source of information on adult and infant mortality?
(A) Disease registers
(B) Vital statistics
(C) Special survey study
(D) Hospital clinic statistics

1. What would be the best source of information on adult and infant mortality?
(B) Vital statistics
Data are routinely collected on all deaths that occur in the US through death certificates. Death certificates record the age at death, and are reported through the vital statistics registration system.
References: Friis and Sellers; pp. 190-191 | Last et al.; p. 187

Which of the following terms is expressed as a ratio (as distinguished from a proportion)?
(A) Male Births / Male + Female Births
(B) Female Births / Male + Female Births
(C) Male Births / Female Births
(D) Stillbirths / Male + Female Births

Which of the following terms is expressed as a ratio (as distinguished from a proportion)?
(C) Male Births / Female Births
A ratio is defined as "the value obtained by dividing one quantity by another; a general term of which rate, proportion, percentage, etc. are subsets."
References: Last et al.; p. 152 | Friis and Sellers; p. 89

Mortality rates by sex in the United States generally show the following sex differences:
(A) Males greater than females
(B) Females greater than males
(C) Males equal to females
(D) Males equal to females in the first years of life

Mortality rates by sex in the United States generally show the following sex differences:
(A) Males greater than females
Males generally have higher all-cause age-specific mortality rates than femalesfrom birth to age 85 and older; in 2003, the ratio of male to female age-specific deaths was 1.4 to 1.
References: Hebel and McCarter; p. 20 | Friis and Sellers; pp. 134-138

Morbidity rates by sex in the United States show the following sex differences:
(A) Males greater than females
(B) Females greater than males
(C) Males equal to females
(D) Males equal to females in the first years of life

Morbidity rates by sex in the United States show the following sex differences:
(B) Females greater than males
This is known as the female paradox: females have lower mortality but higher morbidity rates for acute and chronic conditions.
Reference: Friis and Sellers; p. 136

Case-control studies are among the best observational designs to study diseases of:
(A) High prevalence
(B) High validity
(C) Low case fatality
(D) Low prevalence

Case-control studies are among the best observational designs to study diseases of:
(D) Low prevalence
The case control design is desirable when disease occurrence is rare, becasue if a cohort study were used in such a circumstance, a very large number of people would have to be followed to generate enoigh people with the disease for study.
References: Hebel and McCarter; pp. 106-107, 118 | Friis and Sellers; pp. 243-247 | Kleinbaum et al.; pp. 31-32

Comparison of mortality rates due to cancer of the uterus in users and non-users of supplemental estrogen revealed the following mortality rates per 100,000:
Age Users of estrogen Non-users of estrogen
45-54 3.0 1.0
55-70 17.0 6.0

A valid conclusion derived from the above data concerning mortality among estrogen users is:

(A) The mortality rates for cancer of the uterus are higher in estrogen users than non-users in both age groups studied
(B) A causal relationship is demonstrated between the use of estrogen and incidence of uterine cancer
(C) Mortality from cancer of the uterus rises with age regardless of whether estrogen is used
(D) The mortality rate is lower in non-users than users because the symptoms of uterine cancer are detected earlier in the former group of women

Comparison of mortality rates due to cancer of the uterus in users and non-users of supplemental estrogen revealed the following mortality rates per 100,000:
Age Users of estrogen Non-users of estrogen
45-54 3.0 1.0
55-70 17.0 6.0

A valid conclusion derived from the above data concerning mortality among estrogen users is:

(A) The mortality rates for cancer of the uterus are higher in estrogen users than non-users in both age groups studied
A causal relationship cannot be demonstrated based on the results of a single study; the mortality rate is higher in users for both age groups.
References: Friis and Sellers; pp. 110-112, 132-134 | Hebel and McCarter; pp. 17-22 | Kleinbaum et al.; pp. 64-66

In a large case-control study of pancreatic cancer, 20% of the cases were found to be diabetic at the time of diagnosis, compared to 5% of a control group (matched by age, sex, ethnic group, and several other characteristics) that was examined for diabetes at the same time that the cases were diagnosed. The authors concluded that diabetes played a causal role in the development of pancreatic cancer. This conclusion:
(A) Is correct
(B) May be incorrect because there is no control or comparison group
(C) May be incorrect because of failure to establish the time sequence between the onset of diabetes and pancreatic cancer
(D) May be incorrect because of less complete ascertainment of diabetes in pancreatic cancer cases
(E) May be incorrect because of more complete ascertainment of pancreatic cancer in non-diabetic people

In a large case-control study of pancreatic cancer, 20% of the cases were found to be diabetic at the time of diagnosis, compared to 5% of a control group (matched by age, sex, ethnic group, and several other characteristics) that was examined for diabetes at the same time that the cases were diagnosed. The authors concluded that diabetes played a causal role in the development of pancreatic cancer. This conclusion:
(C) May be incorrect because of failure to establish the time sequence between the onset of diabetes and pancreatic cancer
Case control studies cannot demonstrate the temporal relationship between an exposure and a disease. This is a requirement for concluding causality.
References: Kleinbaum et al.; pp. 22-24 | Hebel and McCarter; pp. 140-141 | Friis and Sellers; p. 72

Cohort study is to risk ratio as:
(A) Ecologic fallacy is to cross-sectional study
(B) Case-control study is to odds ratio
(C) Genetics is to environment
(D) Rate ratio is to ecologic study

Cohort study is to risk ratio as:
(B) Case-control study is to odds ratio
The risk ratio is the measure of association for a cohort study; the odds ratio is the meausre of association for a case-control study.
References: Kleinbaum et al.; pp. 74-75 | Friis and Sellers; pp. 240-241 | Last et al.; p. 128; Hebel and McCarter; p. 111

It is essential in cohort studies of the role of a suspected factor in the etiology of a disease that:
(A) There are equal numbers of people in both study groups.
(B) At the beginning of the study, those with the disease and those without the disease have equal risks of having the factor.
(C) The exposed and non-exposed groups under study should be as similar as possible with regard to possible confounders.
(D) The incidence of the disease is low.

It is essential in cohort studies of the role of a suspected factor in the etiology of a disease that:
(C) The exposed and non-exposed groups under study should be as similar as possible with regard to possible confounders.
This helps ensure comparability of the two groups, increasing the inetrnal validity of the findings. All other answers are false.
Reference: Friis and Sellers; pp. 266-267

A researcher is interested in identifying potential risk factors for a rare form of bone cancer in children. Which of the following study designs would be best suited to investigate the risk factors for this type of cancer?
(A) Prevalence-survey
(B) Case-control
(C) Cohort
(D) Descriptive
(E) Experimental

A researcher is interested in identifying potential risk factors for a rare form of bone cancer in children.
Which of the following study designs would be best suited to investigate the risk factors for this type of cancer?
(B) Case-control
Case-control studies are best suited for rare diseases.
References: Hebel and McCarter; pp. 106-107, 118 | Friis and Sellers; pp. 243-247 | Kleinbaum et al.; pp. 31-32

Which of the following statements regarding disease measures is inaccurate?
(A) Prevalence equals duration time incidence
(B) Prevalence is a proportion
(C) Cumulative incidence is a proportion
(D) Incidence density is a proportion

Which of the following statements regarding disease measures is inaccurate?
(D) Incidence density is a proportion
The numerator for incidence density is "number of cases" and the denominator is "person-time." This represents a rate and not a proportion.
References: Last et al.; p. 92 | Hebel and McCarter; p. 25 | Friis and Sellers; p. 98 | Kleinbaum et al.; pp. 53-54

In a population of 5,000 people, 100 ate spinach contaminated with E. coli (O157:H7) and became ill. Of the ill, 15 died. What was the case fatality rate?
(A) 20 per 1,000
(B) 3 per 1,000
(C) 150 per 1,000
(D) 15 deaths

In a population of 5,000 people, 100 ate spinach contaminated with E. coli (O157:H7) and became ill. Of the ill, 15 died. What was the case fatality rate?
(C) 150 per 1,000
The numerator for case fatality is "number of individuals dying during a specified period of time from the disease of interest" and the denominator is "number of persons with the disease of interest." In this question, the numerator is 15 and the denominator is 100. 15/100 = 0.15 X 1000 = 150/1000 persons.
References: Friis and Sellers; pp. 414-415 | Last et al.; p. 24 | Hebel and McCarter; p. 20 | Kleinbaum et al.; p. 63

A case-control study comparing ovarian cancer cases with community controls found an odds ratio of 2.0 in relation to exposure to radiation. Which is the correct interpretation of the measure of association?
(A) Women exposed to radiation had 2.0 times the risk of ovarian cancer when compared to women not exposed to radiation
(B) Women exposed to radiation had 2.0 times the risk of ovarian cancer when compared to women without ovarian cancer
(C) Ovarian cancer cases had 2.0 times the odds of exposure to radiation when compared to controls
(D) Ovarian cancer cases had 2.0 times the odds of exposure to radiation when compared to women with other cancers

A case-control study comparing ovarian cancer cases with community controls found an odds ratio of 2.0 in relation to exposure to radiation. Which is the correct interpretation of the measure of association?
(C) Ovarian cancer cases had 2.0 times the odds of exposure to radiation when compared to controls.
In a case control study, we are comparing the prevalence of exposure given disease status, therefore, we compare the prevalence of radiation among those with cancer to the prevalence of radiation among those without cancer. We cannot use "risk" to describe this association because risk implies probability and we are calculating odds.
References: Friis and Sellers; pp. 240-241 | Last et al.; p. 128 | Kleinbaum et al.; pp. 74-75

Double-blinded (masked) studies are an important way to:
(A) Achieve comparability of cases and controls
(B) Avoid observer and interviewee bias
(C) Avoid observer bias and sampling variation
(D) Reduce the effects of sampling variation
(E) Avoid interviewee bias and sampling variation

Double-blinded (masked) studies are an important way to:
(B) Avoid observer and interviewee bias
Double masking or blinding a study means that neither the patient nor the individuals involved in evaluating the outcome know the exact exposure status of any individual. This decreases the risk of observer bias because the observer does not know the true exposure. It also decreases the risk of interviewee bias because the participant doesn't know which exposure group he/she is in.
References: Friis and Sellers; pp. 304-305 | Last et al.; p. 18 | Kleinbaum et al.; p. 26 | Hebel and McCarter; p. 125

The difference between primary and secondary prevention of disease is:
(A) Primary prevention focuses on control of causal factors, while secondary prevention focuses on control of symptoms
(B) Primary prevention focuses on control of acute disease, while secondary prevention focuses on control of chronic disease
(C) Primary prevention focuses on control of causal factors, while secondary prevention focuses on early detection and treatment of disease
(D) Primary prevention focuses on increasing resistance to disease, while secondary prevention focuses on decreasing exposure to disease

The difference between primary and secondary prevention of disease is:
(C) Primary prevention focuses on control of causal factors, while secondary prevention focuses on early detection and treatment of disease
Primary prevention suggest that the "incidence" of disease is prevented. In general, this means that the prevention has decreased the prevalence of risk factors or increased the prevalence of preventive factors thus changing the incidence of disease. In secondary prevention, disease has already occurred and the prevention program is designed to identify disease early so that treatment can be started early.
References: Friis and Sellers; p. 80 | Last et al.; pp. 141-142 | Hebel and McCarter; p. 59

A screening test is used in the same way in two similar populations, but the proportion of false-positive results among those who test positive in population B is higher than that among those who test positive in population A. What is the most likely explanation for this finding?
(A) The specificity of the test is higher in population A
(B) The specificity of the test is lower in population A
(C) The prevalence of disease is higher in population A
(D) The prevalence of disease is lower in population A

A screening test is used in the same way in two similar populations, but the proportion of false-positive results among those who test positive in population B is higher than that among those who test positive in population A. What is the most likely explanation for this finding?
(C) The prevalence of disease is higher in population A
The number of false positives increases as the prevalence of the outcome of interest decreases. Communities with lower prevalence of outcome will have higher false-positive rates. I community B has higher false- positive rates, then it must have lower prevalence compared to population A. Therefore, population A has a higher prevalence of disease than community B.
References: Friis and Sellers; pp. 382-385 | Hebel and McCarter; pp. 62-63 | Kleinbaum et al.; pp. 152-154 | Last et al.; p. 140

The relative risk of dying for smokers compared to non-smokers is:
The death rate per 100,000 for lung cancer is 7 among non-smokers and 71 among smokers. The death rate per 100,000 for coronary thrombosis is 422 among non-smokers and 599 among smokers. The prevalence of smoking in the population is 55%.


(A) 9.1 for lung cancer and 0.30 for coronary thrombosis
(B) 9.1 for lung cancer and 1.4 for coronary thrombosis
(C) 10.1 for lung cancer and 8.4 for coronary thrombosis
(D) 10.1 for lung cancer and 1.4 for coronary thrombosis
(E) 12.4 for lung cancer and 1.7 for coronary thrombosis

The relative risk of dying for smokers compared to non-smokers is:
The death rate per 100,000 for lung cancer is 7 among non-smokers and 71 among smokers. The death rate per 100,000 for coronary thrombosis is 422 among non-smokers and 599 among smokers. The prevalence of smoking in the population is 55%.

(D) 10.1 for lung cancer and 1.4 for coronary thrombosis
Prevalence is not relevant to this question - so ignore the prevalence of smoking statement. The relative risk of dying from lung cancer for smokers compared to non-smokers is 71/7 = 10.1. The risk of dying from coronary thrombosis comparing smokers is 599/422 = 1.4.
References: Friis and Sellers; p. 276 | Kleinbaum et al.; pp. 72-73 | Hebel and McCarter; pp. 29, 118 | Last et al.; p. 156

Which measure of mortality would you calculate to determine the proportion of all deaths that is caused by heart disease?
(A) Case fatality
(B) Cause-specific mortality rate
(C) Crude mortality rate
(D) Proportionate mortality ratio
(E) Potential years of life lost

Which measure of mortality would you calculate to determine the proportion of all deaths that is caused by heart disease?
(D) Proportionate mortality ratio
PMR is calculated as the number of deaths within a population due to a specific disease or cause divided by the total number of deaths in the population.
Reference: Friis, page 118

The staff at the state health department Birth Defects Registry systematically visit the labor and delivery units of all hospitals in the state on an on-going basis and review their records to identify major congenital malformations. This is an example of:
(A) Sentinel surveillance
(B) Active surveillance
(C) Passive surveillance
(D) Syndromic surveillance
(E) Reportable conditions

The staff at the state health department Birth Defects Registry systematically visit the labor and delivery units of all hospitals in the state on an on-going basis and review their records to identify major congenital malformations. This is an example of:
(B) Active surveillance
Active surveillance refers to a system in which project staff make periodic visits to clinics and hospitals to identify new cases of disease that have occurred (case finding).
Reference: Gordis, page 54

Which of the following statements is true about surveillance data?
(A) It cannot be used to detect geographical clusters
(B) It includes information on health outcomes, but not exposures or risk factors
(C) It provides early recognition of an infectious disease outbreak, but is much less useful for non-infectious disease events
(D) It can be used to monitor temporal trends in disease
(E) It only includes data pertaining to vaccine-related diseases

Which of the following statements is true about surveillance data?
(D) It can be used to monitor temporal trends in disease
Monitoring temporal trends in disease is a primary function of surveillance systems. While surveillance historically, and perhaps more frequently, is conducted for infectious diseases, in recent years it has become increasingly important in monitoring changes in other types of conditions, such as cancer, congenital malformations, asthma, occupational exposures and diseases, and injuries and illness after natural disasters.
Reference: Gordis page 54

The following are characteristics of a cross-sectional study except:
(D) Patients are followed for a sufficient period of time for outcome events to occur
Patients are not followed in a cross-sectional study.

The following are characteristics of a cross-sectional study except:
(D) Patients are followed for a sufficient period of time for outcome events to occur
Patients are not followed in a cross-sectional study.

All of the following statements are true about both prospective and retrospective cohort studies except:
(A) They measure the incidence of disease
(B) They allow assessment of possible associations between exposure and many outcomes
(C) They require the manipulation of the exposure of interest by the investigator
(D) They avoid bias that might occur if measurement of exposure is made after the outcome of interest is known

All of the following statements are true about both prospective and retrospective cohort studies except:
(C) They require the manipulation of the exposure of interest by the investigator
There is no manipulation of the exposure in a cohort study.

In a cross-sectional study of peptic ulcer among men and women aged 35 to 59, 70 per 100,000 men and 95 per 100,000 women met the criteria for symptomatic peptic ulcer disease. The inference that in this age group, women are at greater risk of developing peptic ulcer is:
(A) Correct
(B) Incorrect because of failure to distinguish between incidence and prevalence
(C) Incorrect because rates were used to compare males and females
(D) Incorrect because of failure to recognize a possible cohort effect
(E) Incorrect because there is no comparison or control group

In a cross-sectional study of peptic ulcer among men and women aged 35 to 59, 70 per 100,000 men and 95 per 100,000 women met the criteria for symptomatic peptic ulcer disease. The inference that in this age group, women are at greater risk of developing peptic ulcer is:
(B) Incorrect because of failure to distinguish between incidence and prevalence
Prevalence does not represent risk.

A prevalence study of depression drew a sample of the residents of Houston by visiting all residence in randomly selected census tracts. A screening test with a sensitivity of 99% and a specificity of 50% was used to identify individuals as "likely depressed." The study reported a prevalence of 28%. This prevalence is:
(A) A true representation of depression in this population
(B) Higher than expected because of the specificity of the screening test
(C) Lower than expected because of the specificity of the screening test
(D) Higher than expected because of the sensitivity of the test
(E) Lower than expected because of the sensitivity of the test

A prevalence study of depression drew a sample of the residents of Houston by visiting all residence in randomly selected census tracts. A screening test with a sensitivity of 99% and a specificity of 50% was used to identify individuals as "likely depressed." The study reported a prevalence of 28%. This prevalence is:
(B) Higher than expected because of the specificity of the screening test
Higher than expected because the specificity is so low there will be many false positives.

Epidemiology could be defined as the study of distribution, determinants,
and control of health-related states or events in the specified population and
the application of this study to control health problems. As an epidemiologist,
if required to fulfill the "distribution" and "determinant" components of
this definition, what will this entail?

a. Public health infrastructure
b. Information, behavior, and health skills development
c. Information dissemination to the population at risk
d. Frequency, patterns, and causes of health -related events
e. Prevention of health -related events

The answer correct is 0 (meaning of epidemiology). Epidemiology refers to
the study of the frequency, patterns, causes, and control and prevention of
diseases, disabilities, and injuries in the human population. There are two
main aspects of epidemiology: (a) distribution-epidemiology evaluates the
distribution of health states by age, gender, race, geography, time, etc., and
(b) epidemiology seeks to examine the association and causal association
between the exposure and the health-related states or events (Last, J.M., A
Dictionary of Epidemiology, 3rd ed., 1995; MacMahan, B, & Trichopoulos, D.,
Epidemiology, Principles & Methods, 2nd ed., 1996, p. 1).

In a survey conducted to examine the prevalence of two diseases, X and Y, in
a specific population, the investigators found that the prevalence of disease
X was higher than the prevalence of disease Y. If the incidence and seasonal
pattern of both diseases are similar, which of these statements are consistent
with this observation?
a. Patients die quickly from disease Ybut not diseaseX
b. Patients die quickly from diseaseXbut not disease Y
c. Patients recover more quickly from disease Ythan from diseaseX
d. Patients recover more quickly from diseaseXthan from diseaseY
e. a and conly

The correct answer isE(factors influencing the prevalence of disease).Prevalence
proportion does not measure disease onset but the disease status.
Whereas incidence is a measure of risk, prevalence is not because it does not
take into account the duration of disease. But prevalence is related to the
duration of the disease and incidence of disease. Prevalence = Incidence x
Disease duration. The number of people living with a disease (prevalence)
will be influenced by the rate at which a new disease develops as well asthe
rate at which they are eliminated through cure (prognosis) or death (mortality).
Therefore, the prevalence will increase if incidence increasesand if there
is improvement in treatment but not cure, and it will decrease if there is dramatic
cure (quick recovery) (Gordis, L., Epidemiology, 3rd ed., 2004, p. 40;
Aschengrau, A.,&Seage, G.R., Essentials ofEpidemiology, 2003, pp. 48-49).

The approach to epidemiology can be described as descriptive, which
involves the analysis of disease, disability, and injury patterns by person,
place, and time. During the summer of 2006, there was an outbreak of bacterial
meningitis in Nocity, Illinois. If you are appointed to investigate this epidemic
using adescriptive epidemiologic approach, what are the main reasons
for describing this outbreak by person, place, and time?
a. To assess the health status of the Nocity population
b. To generate a hypothesis about the causal factors (causative
pathogen/s) for this epidemic
c. To plan and evaluate the public health programs to manage this epidemic
d. a and conly
e. All of the above

The correct answer is E (descriptive epidemiology).

During the third week of January 2006, 80 persons in Newtown of a population
of 450 attended a wedding ceremony, which included a meal prepared
by several of the participants.Within 2 days, 35 of the participants became ill
with salmonellosis. Estimate t he attack rate among the participants.
a. 7.7/100
b. 43.75/ 100
c. 17.7/100
d. 21.2/100
e. None of the above

The correct answer is B (primary attack rate). Attack rate, which is really a proportion
and not a rate, is useful for comparing the risk of disease in groups
with different exposures. This measure is calculated by Number of people at
risk in whom a certain illness developsfTotal number of people at risk. Substituting,
35/80 x (multiplier) = 35/80 x 100 -+ 43.75 per 100 (Gordis, L.,
Epidemiology, 3rd ed., 2004, p. 25).

In a small community of 1,000 households with a population of 5,000 people,
150 persons were affected with E. Coli gastroenteritis in 100 households. A
total of 500 persons lived in the 100 affected households. If each household
had only one primary case, estimate the secondary attack rate.
a. 12.5%
b. 30.0%
c. 10.0%
d. 25.0%
e. 37.5%

The correct answer is A (secondary attack rate). Secondary attack rate yields
the index of the spread of a disease within a family, household, dormitory,
dwelling unit, etc. This measure is estimated by Number of new cases in a
group - Initial case(s) / Number of susceptible persons in a group - Initial
case(s). Substituting: 150 - 100 (each household had one primary case and
there were 100 households) /500 - 100 (primary case) x 100 -+ 50/400 x
100 = 12.5% (Mausner, J.,& Kramer,S., Epidemiology-An Introductory Text,
2nd ed., 1985, pp. 280-281).

In the Framingham Heart Study (FHS), the residentsof the state of Massachusetts
have been followed since the1950s to identify occurrence (heart disease)
and risk factors for heart disease. This study identified high blood
pressure, hyperlipidemia, stress, and physical inactivates as risk factors in
heart disease. This dynamic cohort now involves the grandchildren of the
original cohort. Which of the following study designs represent this study?
a. Case cross-over
b. Ambi-directional cohort
c. Retrospective cohort
d. Case control
e. None of the above

The correct answer is E(study design). A design that begins with exposure(s)
of interest and disease-free subjects and aimed to assess the outcome in the
future by following the subjects represents a prospective cohort study. This
is not a clinical trial or a broad intervention design because the subjects of
the FHS were not assigned but observed. The time relation in a prospective
cohort study is present and future (continuing). A case cross-over design
uses the previous experience of the cases as a substitute for a control series
88 CPH EXAM QUICK REFERENCE REVIEW
to estimate the person-time distribution in the source population. Ambidirectional
cohort isa design tha t is both retrospective and prospective in its
observation of the outcome. Retrospective cohort (historical cohort studies)
is a design in which the cohorts are identified from recorded information and
the time during which they are at risk for disease occurred before the beginning
of the study. Simply, retrospective study is conducted by defining the
cohort and collecting information, which applies to past time. Case control is
a design in which the groups of individuals are defined in terms of whether
they have or have not already exper ienced the outcome under consideration,
and the exposure is then measured (Elwood, J.M., Critical Appraisal of
Epidemiological Studies in Clinical Trials, 2nd ed., 2003, pp. 14-35; Rothman,
K.J., Epidemiology, An Introduction, 2002, p. 89).

A study was conducted in which men (15-49 years) with testicular neoplasm
and a comparable group of men (15-49 years) without testicular neoplasm
were asked about their prior use of steroidal anti-inflammatory drugs. If you
assume that the subjects were selected on the basis of diseaseand not exposure,
and the risk as a measure of association is not directly estimated but
obtained as odds ratio, this is an example of which type of design(s)?
a. Experimenta I
b. Observational
c. Cohort
d. Case control
e. band d only

The correct answer is E (study design). This is an observational design
because it involves natural observation without assignment. A casecontrol is
a design in which the outcome had already occurred prior to the commencement
of the study, and study participants were selected on the basis of the
disease.The direct measure of risk is not possible in case-control study but
estimated as odds ratio.

Epidemiologic designs are largely observational. Often, the critical issue
restricting the conduct of experimental, clinical, or intervention trial is the ethical
consideration such asassigning humans to a known carcinogen (e.g., nicotine).
Which of the following statements illustrate(s) the distinction between
the two designs?
a. Investigator is"blinded" (prevented from knowing the subjects' true
exposure status until the end of the study) in an experimental study but
not in an observational study
b. Investigator controls the subject's outcome in an experimental study
but not in an observational study
c. Investigator controls the subject's exposure in an experimental study
but not in an observational study
d. Experimental studies are conducted with animals; observational studies
are conducted with humans
e. a and conly

The correct answer is E (observational versus experimental design). Experimental
design if feasible is considered the gold standard compared with
observational studies. The active manipulation or assignment of the treatment
by the investigator is the hallmark that differentiates experimental
designs from observational studies. Whereas not all experimental designs
utilize blindness to minimize bias, blindness is not a feature of observational
designs. Neither in observational design nor experimental ones are investigators
required to manipulate the outcome. Experimental designs, like
observational ones, are conducted in human as well as animal populations
(Aschengrau, A.,& Seage, G.R., EssentialsofEpidemiology, 2003, pp. 163-165).

Confounding is the mixing effect of an extraneous variable in the association
between the predictor (independent variable) and outcome (dependent
variable). A study was conducted to examine the association between pancreatic
cancer and coffee drinking. If a third variable, cigarette smoking, is
assumed to be a confounder in this association, which of the following features
will qualify smoking as a confounder in this association?
a. Smoking is associated with coffee drinking in the population that produced
the cases
b. Smoking is an independent cause or predictor of pancreatic cancer
c. Smoking is not an intermediate step in the causal pathway between
coffee drinking and pancreatic cancer
d. a and conly
e. All of the above

The correct answer is E (internal validity of study). Confounding is derived
from Latin "coniondere", meaning lito pourtogether;' and it indicates the conCHAPTER
2:PRINCIPLESAND METHODSOF EPIDEMIOLOGY 89
fusion of two supposedly causal variables so that part or all of the purported
effect of one variable (exposure) is actually due to the other (confounder).
Confounding variable is considered an extraneous variable because it competes
with the independent variable in explaining the outcome. In a study
conducted to assess whether exposure (A) causesdisease (8),Cis a confounding
if (a) C (smoking) is associated with A (coffee drinking) in the population
that produced the cases 8 (pancreatic cancer), (b) C(smoking) is an independent
cause or predictor of 8 (pancreatic cancer), or (c) C (smoking) is not an
intermediate step in the causal pathway between coffee drinking (A) and pancreatic
cancer (8) (Hennekens, C.H., & Buring, J.E., Epidemiology in Medicine,
1987, pp. 287-293; Gordis, L., Epidemiology, 3rd ed., 2004, pp. 228-229).

Clinicians may examine men who are diabetic more often or more thoroughly
than men who are not. If this observation is accurate, and if an association
is observed between erectile dysfunction (ED) and diabetes mellitus
in men, the association may be due to which of the following bias?
a. Misclassification bias
b. Recall bias
c. Surveillance bias
d. Interviewer bias
e. None of the above
••.•.......•.........•...................•....•.....••...•••.••••...••••••••....•.••••.•.•••......•

The correct answer is C (study validity-bias). Because men with diabetes
mellitus are more heavily monitored compared to men who are not, disease
ascertainment may be better in the monitored population and may introduce
surveillance bias (monitoring of one population [diseased] more than
the other [disease-free]). This bias may lead to erroneous estimation of the
effect of the disease in the monitored population, thus inflating the point
estimate (RR, OR) (Gordis, L., Epidemiology, 3rd ed., 2004, p. 227).

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