1. Which of the following is characteristic of a single-exposure, common-vehicle
outbreak? - The epidemic curve has a normal distribution when plotted
... [Show More] against
the logarithm of time
Explanation: Single-exposure, common-vehicle outbreaks involve a sudden,
rapid increase in cases of disease that are limited to persons who share a
common exposure. Additionally, few secondary cases develop among persons
exposed to primary cases. A histogram of the outbreak can plot the number of
cases by time of disease onset. In single-exposure, common-vehicle outbreaks,
a log transformation of the time of disease onset will often take on the
characteristic shape of a normal distribution (i.e., a bell curve) with the median
incubation time found at the peak of the curve.
2. A group of researchers are interested in conducting a clinical trial to determine
whether a new cholesterol-lowering agent was useful in preventing coronary
heart disease (CHD). They identified 12,327 potential participants for the trial. At
the initial clinical exam, 309 were discovered to have CHD. The remaining
subjects entered the trial and were divided equally into the treatment and placebo
groups. Of those in the treatment group, 505 developed CHD after 5 years of
follow-up while 477 developed CHD during the same period in the placebo group.
What was the prevalence of CHD at the initial exam? - The prevalence of CHD at
the initial exam was 309 cases of CHD divided by 12,327 participants. This
equals a prevalence of 25.1 cases of CHD per 1,000 persons.
3. A group of researchers are interested in conducting a clinical trial to determine
whether a new cholesterol-lowering agent was useful in preventing coronary
heart disease (CHD). They identified 12,327 potential participants for the trial. At
the initial clinical exam, 309 were discovered to have CHD. The remaining
subjects entered the trial and were divided equally into the treatment and placebo
groups. Of those in the treatment group, 505 developed CHD after 5 years of
follow-up while 477 developed CHD during the same period in the placebo group.
What was the incidence of CHD during the 5-year study? - The incidence rate
reflects the number of new cases developing in the population at risk. Since
prevalent CHD cases were excluded from the study, the population at risk was
12,018 (12,327 persons less 309 cases of CHD). During the 5-year study period,
982 incident cases of CHD developed. This equals an incidence rate of 81.7
cases of CHD per 1,000 persons.
4. Which of the following are examples of a population prevalence rate?
A) The number of ear infections suffered by 3-year-old children in March,
2006B) The number of persons with hypertension per 100,000 population
C) The number of cases of skin cancer diagnosed in a dermatology clinic
D) b and c
E) All of the above - B) The number of persons with hypertension per
100,000 population
Explanation: Prevalence is the number of affected persons in a specified
population size at a given time. Only answer (b) fits this definition. Example (a) is
more consistent with an incident rate while answer (c) is a selected group of
persons who may not be representative of a general population.
5. What would be the effect on age-specific incidence rates of uterine cancer if
women with hysterectomies were excluded from the denominator of incidence
calculations assuming that most women who have had hysterectomies are older
than 50 years of age.
A) The rates in all age groups would remain the same.
B) Only rates in women older than 50 years of age would tend to decrease.
C) Rates in women younger than 50 years would increase compared to
women older than 50 years of age.
D) Rates would increase in women older than 50 years of age but may
decrease in younger women as they get older.
E) It cannot be determined whether the rates would increase or decrease. -
D) Rates would increase in women older than 50 years of age but may
decrease in younger women as they get older.
Explanation: Women who have had hysterectomies (i.e., removal of the uterus)
are no longer at risk for uterine cancer. For women older than 50 years of age,
this would increase the age-specific incidence rate as there would be the same
number of uterine cancers occurring among fewer women at risk. Further, rates
may decrease among younger women who have had hysterectomies as they are
no longer at risk for uterine cancer and thus may decrease the number of
potential cases occurring in their age group over time.
6. A prevalence survey conducted from January 1 through December 31, 2003
identified 580 new cases of tuberculosis in a city of 2 million persons. The
incidence rate of tuberculosis in this population has historically been 1 per 4,000
persons each year. What is the incident rate of tuberculosis per 100,000 persons
in 2003?
- The answer is 29 new cases of tuberculosis per 100,000 persons. This is found
by dividing the new cases of tuberculosis by the total population at risk
(580/2,000,000) and multiplying this rate by 100,000 to standardize the rate.
7. A prevalence survey conducted from January 1 through December 31, 2003
identified 580 new cases of tuberculosis in a city of 2 million persons. The
incidence rate of tuberculosis in this population has historically been 1 per 4,000
persons each year. Has the risk of tuberculosis increased or decreased during
2003?- The risk of tuberculosis has increased over the historic incident rate. This
comparison can be made by standardizing the historic rate to a rate per 100,000
persons. To do this, multiply the numerator and denominator by 25.
8. Which of the following is an advantage of active surveillance?
A) Requires less project staff
B) Is relatively inexpensive to employ
C) More accurate due to reduced reporting burden for health care providers
D) Relies on different disease definitions to account for all cases
E) Reporting systems can be developed quickly
- C) More accurate due to reduced reporting burden for health care providers
Explanation: Active surveillance entails a concerted effort to collect information
about disease occurrence. It typically involves dedicated staff members who
have been specifically directed to contact physicians and hospitals in order to
collect reports of disease cases in a specified population. This activity requires a
large amount of staff and resources in order to accomplish its goals.
9. The population of a city on February 15, 2005, was 36,600. The city has a
passive surveillance system that collects hospital and private physician reports of
influenza cases every month. During the period between January 1 and April 1,
2005, 2,200 new cases of influenza occurred in the city. Of these cases, 775
persons were ill with influenza according to surveillance reports on April 1, 2005.
The monthly incidence rate of active cases of influenza for the 3-month period
was:
- C) 20 per 1,000 population
Explanation: The monthly incidence rate is calculated based on the number of
new cases of a disease developing during the 3-month time period. In this
example, 2,200 cases of influenza developed among an average population of
36,600 persons at risk during the surveillance period. The incidence rate equals
2,200 divided by 36,600. In order to calculate the average monthly rate, the rate
should then be divided by 3. Finally, the monthly rate can be multiplied by 1,000
in order to express it per the responses listed.
10.The population of a city on February 15, 2005, was 36,600. The city has a
passive surveillance system that collects hospital and private physician reports of
influenza cases every month. During the period between January 1 and April 1,
2005, 2,200 new cases of influenza occurred in the city. Of these cases, 775
persons were ill with influenza according to surveillance reports on April 1, 2005.
The prevalence rate of active influenza as of April 1, 2005, was:
A) 10 per 1,000 population
B) 14 per 1,000 population
C) 17.5 per 1,000 population
D) 20 per 1,000 population
E) Cannot be calculated as there is no information on duration
- D) 20 per 1,000 populationExplanation: The prevalence rate as of April 1, 2005, is equal to the number of
active influenza cases reported divided by the number of persons at risk in the
population at that time. The best estimate of the population size is that from the
February 15 count, less who are no longer at risk as they have already recovered
from influenza and have developed immunity. Therefore, prevalence equals 705
cases divided by 36,600 less 1,495 recovered cases. This number can be
multiplied by 1,000 in order to estimate a prevalence rate of 20 cases per 1,000
persons. [Show Less]