Epidemiology Practice Quiz

1. Epidemic refers to

A disease that has a low rate of occurrence but that is constantly present in acommunity or region

An attack rate in excess of 10 per 1,000 population

The occurrence of illnesses of similar nature clearly in excess of the normal expectation for that population at that time

Diseases of the respiratory system that occur seasonally

The annual case rate per 100,000 population

Epidemics (or outbreaks) are said to occur when the number of cases of a disease or event are clearly in excess for what is to be expected in that population at that point in time.

Explanation

Epidemics (or outbreaks) are said to occur when the number of cases of a disease or event are clearly in excess for what is to be expected in that population at that point in time.

2. An outbreak of gastritis occurred on a cruise ship. The data in the following table were obtained shortly after the outbreak, from a questionnaire completed by everyone on board the ship.

What is the relative risk of developing gastritis from herring consumption?

0.5

2.0

2.3

8.0

1.0

Relative risk (RR) = Risk in exposed/Risk in unexposed
And,
Risk = Number of cases (d)/ Total number of people at risk (N)
Therefore,
RR for developing gastritis from herring consumption = Risk in ‘Ate’/ Risk in ‘Did not eat’
= (200/1000)/(100/1000)

Explanation

Relative risk (RR) = Risk in exposed/Risk in unexposed
And,
Risk = Number of cases (d)/ Total number of people at risk (N)
Therefore,
RR for developing gastritis from herring consumption = Risk in ‘Ate’/ Risk in ‘Did not eat’
= (200/1000)/(100/1000)

3. In 1945, there were 1,000 women who worked in a factory painting radium dials on watches. The incidence of bone cancer in these women up to 1975 was compared to that of 1,000 women who worked as telephone operators in 1945. Twenty of the radium dial painters and four of the telephone operators developed bone cancer between 1945 and 1975. This is an example of a

Cohort study

Experimental study

Clinical trial

Cross-sectional study

Case-control study

The two groups where selected on the basis of the exposure (radium) and
followed over time (1945 to 1975) to see what the outcome experience (bone cancer)
was. In fact because the investigators when back in time to establish the groups this
study could also be called a retrospective cohort study (as compared to the usual
prospective cohort study in which the groups are established in real time and followed
up.

Explanation

The two groups where selected on the basis of the exposure (radium) and
followed over time (1945 to 1975) to see what the outcome experience (bone cancer)
was. In fact because the investigators when back in time to establish the groups this
study could also be called a retrospective cohort study (as compared to the usual
prospective cohort study in which the groups are established in real time and followed
up.

4. Regionville is a community of 100,000 persons. During 1985 there were 1,000 deaths from all causes. All cases of tuberculosis have been found and they total 300 – 200 males and 100 females. During 1985, there were 60 deaths from tuberculosis, 50 of them males. The crude mortality rate for Regionville is

300 per 100,000

60 per 1,000

10 per 1,000

100 per 1,000

200 per 1,000

The crude mortality rate it the death rate that is calculated using all
deaths as the numerator and the total population as the denominator. Therefore
Crude mortality rate = total deaths/total population
= 1000/100,000
= 10 per 1,000

Explanation

The crude mortality rate it the death rate that is calculated using all
deaths as the numerator and the total population as the denominator. Therefore
Crude mortality rate = total deaths/total population
= 1000/100,000
= 10 per 1,000

5. A screening test for breast cancer was administered to 400 women with biopsy-proven breast cancer and to 400 women without breast cancer. The test results were positive for 100 of the proven cases and 50 of the normal women. The sensitivity of the test is

88%

67%

25%

33%

12%

Sensitivity = (# correctly positive on screening test/ # positive on gold standard)x 100
= (100/400) x 100
= 25%

Explanation

Sensitivity = (# correctly positive on screening test/ # positive on gold standard)x 100
= (100/400) x 100
= 25%

6. California Highway Patrol statistics revealed that more accidents occurred to blue cars than to cars of any other color. The inference that while driving a blue car, one is at higher risk of an accident than while driving a car of another color is

Correct

Incorrect, because the comparison is not based on rates

Incorrect, because no control or comparison group is used

Incorrect, because no test of statistical significance has been made

Incorrect, because prevalence is used instead of incidence

The reason that the number of blue cars involved in accidents may be
simply because there are more blue cars than other color cars driving on that particular
highway. The correct approach is compare the rate (or risk) at which blue cars have
accidents against the rate at which other colored cars have accidents. This involves
calculating the number of accidents per X number of _ color car.

Explanation

The reason that the number of blue cars involved in accidents may be
simply because there are more blue cars than other color cars driving on that particular
highway. The correct approach is compare the rate (or risk) at which blue cars have
accidents against the rate at which other colored cars have accidents. This involves
calculating the number of accidents per X number of _ color car.

7. A screening test for breast cancer was administered to 400 women with biopsy-proven breast cancer and to 400 women without breast cancer. The test results were positive for 100 of the proven cases and 50 of the normal women. The predictive value of a positive test is

88%

67%

25%

33%

12%

PPV = (# positive on screening test and gold std/ Total # positive by screening test)x 100
= (100/150) x 100
= 67%

Explanation

PPV = (# positive on screening test and gold std/ Total # positive by screening test)x 100
= (100/150) x 100
= 67%

8. A screening test for breast cancer was administered to 400 women with biopsy-proven breast cancer and to 400 women without breast cancer. The test results were positive for 100 of the proven cases and 50 of the normal women. The specificity of the test is

88%

67%

25%

33%

12%

Specificity = # correctly identified as neg by screening test/ # negative on gold std.)x 100
= (350/400) x 100
= 88%

Explanation

Specificity = # correctly identified as neg by screening test/ # negative on gold std.)x 100
= (350/400) x 100
= 88%

9. In a study of 500 cases of a disease and 500 controls, the suspected etiological factor is found in 400 cases and 100 controls. The absolute risk of disease in people with the factor is

80%

40%

16%

20%

Cannot be computed from the data given

Only odds are calculated as a measure of incidence from case-control studies. In a case control study one is not sure about all the persons who had the particular exposure at the time in question and therefore the total number of persons at risk is unknown. Likewise, the duration of exposure to the risk factor in most instances and therefore rates cannot be calculated. The odds however give a measure that is numerically close to the risk.

Explanation

Only odds are calculated as a measure of incidence from case-control studies. In a case control study one is not sure about all the persons who had the particular exposure at the time in question and therefore the total number of persons at risk is unknown. Likewise, the duration of exposure to the risk factor in most instances and therefore rates cannot be calculated. The odds however give a measure that is numerically close to the risk.