EPIDEMIOLOGY-BIOSTATISTICS EXAM

Midterm 2004

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Instructions: This exam is 20% of your course grade. The maximum number of points

for the course is 1,000; hence, this exam is worth 200 points. There are 20 questions on

this exam. Each question is worth 10 points to yield the maximum of 200 points for this

exam. For questions 1-10, record the best answer in pencil on the answer sheet provided.

For questions 11-20, write your answers in the spaces provided. Submit your exam and

your answer sheet as directed after you have completed the exam.

1. Investigators want to know if a new diet can reduce the risk of stroke in adult

women. They conduct a RCT assigning one arm to the new diet and the other

arm to their routine diet. It is known that smoking and hypertension are strong

risk factors for stroke. Table 1 appears below:

New Diet Routine Diet

(n=500) (n=500)

% Smokers 25 40

% Hypertension 10 10

% family history of stroke 3 3

Select the best answer:

a.) The lack of adult men in the trial affects its internal validity.

b.) It is possible that the crude relative risk for new diet vs. routine diet is 1.0.

c.) The authors should adjust for hypertension because it is a potential confounder in

this trial.

d.) The authors should make the two arms comparable for smoking by matching.

e.) The smoking-adjusted relative risk for new diet vs. routine diet must be closer to

1.0 than the crude relative risk.

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2. Researchers want to study Paroxysmal Nocturnal Hemoglobinuria (PNH), a

disease with an incidence of about 1 per million per year, which manifests itself

during the early adult years. They wonder if there is an association between low

birth weight and the later development of PNH. The researchers should conduct

a: (Select the best answer.)

a.) RCT with low birth weight and normal birth weight as the two arms.

b.) Retrospective cohort study with PNH as one arm and no PNH as the other

arm.

c.) Retrospective cohort study with low birth weight as one arm and normal

birth weight as the other arm.

d.) Case control study with PNH as cases and subjects without PNH as

controls

e.) Large case series

3. Public health officials believe that they could prevent 10 cases of Hepatitis C per

5,000 interns over a 10-year period by preventing all of their needle stick injuries.

Select the best answer:

a.) This is the attributable risk of Hepatitis C due to needle stick injuries in

interns.

b.) This is the incidence of Hepatitis C in interns with needle stick injuries

divided by the incidence of Hepatitis C in interns without needle stick

injuries.

c.) This is the cumulative incidence of Hepatitis C in interns with needle stick

injuries.

d.) This is the incidence rate of Hepatitis C in interns with needle stick

injuries.

e.) This is the prevalence of Hepatitis C in interns with needle stick injuries.

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4. A RCT of 10,000 adults was conducted to determine if a new anti-platelet

medication taken with aspirin is better than aspirin taken alone in subjects with

heart disease. The outcome of interest was MI (heart attack). Select the best

answer:

a.) It is possible that confounding by the new anti-platelet medication could

occur.

b.) Comparing one arm taking two medications to another arm taking only

one medication will likely lead to interaction by the arm taking the two

medications.

c.) After randomization, the arms will likely be comparable for unknown

potential confounders.

d.) Table 1 with the baseline characteristics of both study arms should show

both the known and unknown potential confounders.

e.) Confounding is eliminated by the choice of a RCT.

5. Which statement best describes adjustment? Select the best answer.

a.) Its a method to assure there is an equal distribution of potential

confounders in study arms.

b.) Its used when there is a concern that confounding may lead to an over

estimate or an under estimate of the appropriate measure of association.

c.) It can offset the effects of subjects not adhering to their assigned

protocols.

d.) Its a study design method to help prevent confounding.

e.) Its useful in calculating interaction.

6. Select the best statement concerning recall bias:

a.) Its a bias unique to retrospective cohort studies.

b.) It occurs in RCTs when subjects do not recall their assigned protocols.

c.) It occurs if the outcome assessment committee is not blinded such that

they can recall to which arm the subject was assigned.

d.) It occurs when cases and controls know their outcome status and

differentially recall their exposure history in a case control study.

e.) It occurs when subjects in a case control study differentially recall their

disease outcome.

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7. The Central Limit Theorem (CLT) says that if many random samples of sufficient

size are taken from a population, and the means from each sample are calculated

and plotted, that the plot will have certain characteristics. Select the best

statement.

a.) The CLT predicts that the mean of the distribution of sample means will

be the population mean.

b.) The CLT predicts that the plot of the sample means will be skewed if the

population is skewed.

c.) The standard deviation of the sampling distribution is only dependent on

the standard deviation of the population distribution.

d.) The standard deviation of the population distribution is called the standard

error of the mean.

e.) The standard deviation of the population distribution is interchangeable

with the term standard error.

8. Select the best statement:

a.) The p value can be used to help predict the probability that the null

hypothesis is false.

b.) A trials power equals the probability of a Type II error.

c.) A study result with a p value less than alpha must be statistically

significant.

d.) A study result with a p value less than alpha must be clinically significant.

e.) Performing multiple comparisons without adjusting alpha increases the

probability of a Type II error.

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9. A researcher is conducting a prospective cohort study to determine if there is an

association between high fiber diets and gallbladder cancer. She is the only

investigator and she is not blinded. There is, however, an independent outcomes

committee responsible for determining whether a subject develops gallbladder

cancer during the study. The researcher is very concerned that subjects not on a

high fiber diet might develop colon cancer, a secondary outcome of the study, so

she follows them more closely for colon cancer vs. the subjects on the high fiber

diet. Select the best statement:

a.) By following the subjects not on a high fiber diet more closely, the

researcher will eliminate the chance of non-random misclassification of

gallbladder cancer.

b.) The study is subject to reporting bias if she erroneously estimates the

relative risk of colon cancer in subjects on a high fiber diet compared to

those that are not.

c.) This study is prone to interviewer bias if the researcher misclassifies the

amount of fiber in the subjects diets.

d.) Selection bias could have occurred if the researcher only selected subjects

from a narrow geographical area.

e.) Surveillance bias could lead to non-random misclassification of colon

cancer.

10. Select the correct statement regarding societal determinants of health.

a.) USA citizens have the longest life expectancy of any nation because the

USA spends the most money per capita on health care of any nation.

b.) Discrepancies of health status between industrialized nations can partly be

explained by variations in the social cohesiveness of its citizens.

c.) Once the vast majority of a countrys population is lifted out of poverty,

all of its citizens experience roughly the same health status.

d.) Physicians spend most of their time managing the societal determinants of

their patients health.

e.) The stressor is a more powerful health determinant than the stress

response.

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11. What is meant by an intention-to-treat analysis?

12. How does non-random misclassification bias of outcomes affect study results?

13. A physician initially wants 80% power to detect a true difference of at least 20%

between two study arms with alpha set at 0.05. After further thought, she wants

to achieve 90% power to detect a true difference of at least 20%, still keeping

alpha at 0.05. What does she need to do?

14. An author reports that the relative risk of adults having a heart attack while on

aspirin compared to placebo is 0.8. This result is statistically significant. The

author further reports that the relative risk of skiers having a heart attack while on

aspirin compared to skiers on placebo is 0.7. The relative risk of non-skiers

having a heart attack while on aspirin compared to non-skiers on placebo is 0.5.

The difference between 0.7 and 0.5 is not statistically significant. The author

reports that skiing is an effect modifier in this study. Is the author correct?

Answer yes or know and then state your reasons to support your answer.

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15. What is the essential study design difference between a case control study and a

retrospective cohort study?

16. The test scores from an exam are normally distributed with a mean of 80%. One

standard deviation was 5%. What percent of those taking the exam scored

between 75% and 90%?

17. List the two necessary conditions for confounding to occur.

18. What does it mean when an investigator reports that her study result is statistically

significant?

19. The 95% confidence interval on a sample mean extends from 5.0 to 10.0 Is it

possible that the population mean from which the sample was drawn, the parameter,

does not lie somewhere between 5.0 and 10.0? Explain your answer.

20. List two advantages of a prospective cohort study compared to a case control

study.

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