Understanding Disease Occurrence: Measures, Rates, and Risk Factors in Epidemiology

Question I. During a study of 20 years five people are followed to measure the occurrence of
upper
respiratory tract infection. As this infection can occur more than once, all disease events are
included in this study.
1 person is lost to follow-up after 1.5 years.
2 persons died respectively after 10 and 15 years from a different cause.
1 person got the first respiratory tract infection after seven years and the second infection after 12
years of follow-up. Both infections take half a year of recovery. This person is followed-up until
the end of the study.
One person is followed-up the whole period without occurrence of disease.
1) Which measure of disease occurrence can be calculated?
Incidence rate
2) Calculate measure of disease occurrence.
Incid ence rate= No . of New Cases of Disease∈ a given time period
Total Person−time at risk duringthe follow−uo period
Cases Years at
Risk
perso
n 1
0
1.5
perso
n 2
0
10
perso
n 3
0
15
perso
n 4
2
19
perso
n 5
0
20
2
65.5
Incidenc
e rate
0.0305343
51
Q U E S T I O N 2
Attack rate is a measure of risk. In a small community of 460 residents, 87 attended a social
event that included a meal prepared by several individuals. Within 3 days, 39 of those who

attended the event became ill with a condition diagnosed as salmonella enterocolitis. The attack
rate among attendees was:
11.8/100
8.5/100
18.9/100
44.8/100
Question 3
To compare the risk of Disease A in males vs. the risk in females, which occurrence of disease
we should use and why? (explain your answer)
Category-specific rates
It allows for the assessment of occurrence with regard to a specific cohort or group of individuals
in a population
Question 4
How does prevalence of a disease decrease? When there is a negative change in the number of
new cases of disease in a given population
Provide 4 conditions that can decrease the prevalence of a disease:
1. Disease is cured in infected person
2. Infected person dies
3. Persons are lost to follow-up
4. Persons die due to other diseases or cases
Question 5
A city contains 100,000 people (60,000 males and 40,000 females). During a one year period,
there were 1,000 deaths in the city (600 in males and 400 in females).
The crude mortality rate for the city (per 100,000)
was:
300 x 100,000 = 300 per 100,000
100,000
600 x 100,000 = 600 per 100,000

100,000
45 x 100,000 = 45 per 100,000
100,000
1000 x 100,000 = 1000 per 100,000
100,000
Question 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
accident than driving a car of another color is:
Incorrect
Incorrect, because the comparison is not based on rates
Incorrect, because no test of statistical significance has been made
Incorrect, because prevalence is used instead of incidence
Question 7
This Table describes the number of illness and deaths caused by plague in four communities.
Which community has the lowest case fatality rate for plague?
1.
Community A 100/150=66.7%
Community B 300/400=74%
Community C 300/500=60%
Community D 500/650=76.9%
Question 8
Total
Deaths
Death from
Plague
Sick from
Plague
Community A 200 100 150
Community B 400 300 400
Community C 800 300 500
Community D 5,000 500 650