Epi Questions
Which of the following statements are true? More than one answer may be correct. A) A mortality rate is an example of an incidence rate B) Death certificate data are generally valid regardless of the cause of death C) Type of disease is the most important predictor of mortality D) Changing diagnostic criteria does not affect estimates of prevalence and incidence E) The case-fatality rate is calculated based on the entire population at risk
A) A mortality rate is an example of an incidence rate Explanation: A mortality rate can approximate an incidence rate under conditions of a high case-fatality rate and a short duration of disease
Which of the following statements best describe efficacy? A) It is an estimate of the benefit of treatment under ideal conditions Explanation: Efficacy involves the estimation of treatment benefits, i.e., reduction of disease. It can be estimated for assumed conditions of treatment and typically expresses the reduction in the proportion of those developing the disease when treated with agents such as vaccines. A) It is an estimate of the benefit of treatment under ideal conditions B) It is an estimate of the benefit of treatment under routine conditions C) It is an estimate of the reduction of disease in treated groups D) It is an estimate of the reduction of disease in the population E) It depends on the prevalence of disease
A) It is an estimate of the benefit of treatment under ideal conditions Explanation: Efficacy involves the estimation of treatment benefits, i.e., reduction of disease. It can be estimated for assumed conditions of treatment and typically expresses the reduction in the proportion of those developing the disease when treated with agents such as vaccines. C) It is an estimate of the reduction of disease in treated groups Explanation: Efficacy involves the estimation of treatment benefits, i.e., reduction of disease. It can be estimated for assumed conditions of treatment and typically expresses the reduction in the proportion of those developing the disease when treated with agents such as vaccines.
Which of the following characteristics indicate that mortality rates provide a reliable estimate of disease incidence? More than one answer may be correct. A) Case-fatality rate is low B) The case-fatality rate is high C) The duration of disease is short D) The prevalence of disease is greater than 5% E) The proportionate mortality is high
B) The case-fatality rate is high Explanation: Mortality rates measure the number of deaths (from all causes or specific diseases) occurring in a population. They approximate incidence rates when the number of persons developing and subsequently dying from the disease is high. These aspects are present when the case-fatality rate is high and correspondingly the duration of the disease is short. C) The duration of disease is short Explanation: Mortality rates measure the number of deaths (from all causes or specific diseases) occurring in a population. They approximate incidence rates when the number of persons developing and subsequently dying from the disease is high. These aspects are present when the case-fatality rate is high and correspondingly the duration of the disease is short.
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, 2006 B) 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.
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.
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.
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. What can be inferred about influenza cases occurring in the city? A) Active surveillance would enable better prevention of influenza B) The incidence rate would decrease if active surveillance were employed C) The average duration of influenza is approximately 1 month D) The actual number of influenza cases occurring in the population is less since hospitals and private physicians may be reporting the same patients. E) The prevalence rate should be higher since it should be calculated based on all cases of influenza occurring from January 1 through March 30, 2005.
C) The average duration of influenza is approximately 1 month Explanation: Since the average monthly incidence rate is 20 per 1,000 and the prevalence rate is also 20 per 1,000, then the duration of disease must equal 1 month.
The prevalence rate of a disease is two times greater in women than in men, but the incidence rates are the same in men and women. Which of the following statements may explain this situation? A) The duration of disease is shorter in women B) Men are at greater risk for developing the disease C) The case-fatality rate is lower for women D) The age-adjusted mortality rate will be higher for women E) The proportionate mortality rate for the disease is higher for men
C) The case-fatality rate is lower for women Explanation: Since men and women develop the disease at the same rate, the survival rate in women must be increased in order to increase duration and prevalence. A low case-fatality rate would contribute to an increased duration of the disease.
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 population Explanation: 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.
A study found that adults older than age 50 had a higher prevalence of pneumonia than those who were younger than age 50. Which of the following is consistent with this finding? A) Younger adults have a higher incidence of pneumonia B) Older adults have a higher case-fatality rate from pneumonia C) Younger adults with pneumonia are more likely to report being ill than older persons D) Incidence rates do not vary by age, but older adults have pneumonia for a longer duration compared to younger adults E) None of the above
D) Incidence rates do not vary by age, but older adults have pneumonia for a longer duration compared to younger adults Explanation: For prevalence to be higher among older adults, either incidence or duration of pneumonia must be increased in this age group.
Which of the following statements are true? More than one answer may be correct. A) Prevalence rates are always larger than incidence rates B) In a steady state, the prevalence of disease is equal to the attack rate C) Diagnostic criteria rarely impact estimates of disease prevalence and incidence D) Prevalence rates are useful for public health planning E) Incidence rates can be used to estimate prevalence when the mean duration of the disease is known
D) Prevalence rates are useful for public health planning Explanation: Prevalence rates reflect the burden of illness in a population at a specific time. This is necessary information in order to plan for public health. Further, the prevalence of disease is a function of incidence and mean duration. When both of these are known, then prevalence can be estimated. E) Incidence rates can be used to estimate prevalence when the mean duration of the disease is known Explanation: Prevalence rates reflect the burden of illness in a population at a specific time. This is necessary information in order to plan for public health. Further, the prevalence of disease is a function of incidence and mean duration. When both of these are known, then prevalence can be estimated.
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.
For a disease such as liver cancer, which is highly fatal and of short duration, which of the following statements is true? Choose the best answer. A) Mortality rates will be much higher than incidence rates B) Mortality rates will be much higher than prevalence rates C) Incidence rates will be much higher than mortality rates D) Case-fatality rates will be equal to mortality rates E) Incidence rates will be equal to mortality rates
E) Incidence rates will be equal to mortality rates Explanation: Since the 5-year survival rate for liver cancer is 4%, most incident cases of liver cancer will result in a premature mortality. In this case, the mortality and incidence rates will be approximately equal.
A school nurse examined a population of 1,000 children in an attempt to detect nearsightedness. The prevalence of myopia in this population is known to be 15%. The sensitivity of the examination is 60% and its specificity is 80%. All children labeled as "positive" (i.e., suspected of having myopia) by the school nurse are sent for examination by an optometrist. The sensitivity of the optometrist's examination is 98% and its specificity is 90%. What is the positive predictive value (PPV) of the school nurse's exam?
The PPV of the school nurse's exam is equal to the number of true positive cases divided by the total number of those that the school nurse labels as positive. In this exam, the PPV is 34.6% (90 true myopic children divided by 260 children labeled as myopic by the school nurse).
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.
In a coastal area of a country in which a tsunami struck, there were 100,000 deaths in a population of 2.4 million for the year ending December 31, 2005. What was the all-cause crude mortality rate per 1,000 persons during 2005?
The answer is 41.7 per 1,000 persons. The rate is calculated by dividing 100,000 deaths by the population of 2,400,000 persons. To express as a rate per 1,000 persons, the rate is multiplied by 1,000.
Which of the following is characteristic of a single-exposure, common-vehicle outbreak?
The epidemic curve has a normal distribution when plotted 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.
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.
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.
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.
