This document discusses various epidemiological measures used to describe the frequency and distribution of diseases in populations. It defines incidence as the number of new cases in a population during a specified time period. Incidence rate is calculated by dividing the number of new cases by the total population at risk. Prevalence refers to existing cases in the population at a given time. Incidence density accounts for person-time at risk and divides new cases by total person-years observed. Measures of association like relative risk and odds ratio are used to compare disease frequency between exposed and unexposed groups.

1. Measures of frequency and
association in Epidemiology `
Dr Tumwebaze Mathias PhD
MPH Notes- 2013

2. Incidence
Measures the frequency of new cases of disease or condition in a population at
risk during a specified period of time
Number of new cases of a disease
during a specified period of time
CI= x multiplier ((e.g., 100,000)
Total population at risk during during that time
Assumption: the entire population at the beginning of the study has been followed for the
entire interval for the development of the outcome of interest

3. Risk
• Cumulative incidence is same as
risk.
• If 10/100 over 10 years get a
brain tumour then the risk for
developing a brain tumour in that
population is 10%.

4. Incidence Density
– Incidence Density (ID) – measure of the true rate
of disease development
– True force of morbidity or mortality
new cases in a given period of time
ID = total person-time of observation
(typically person years)

5. Calculating person-time
ID 1/95 1/96 1/97 1/98 1/99 1/00 Total
A 3
B 3
C 5
D 4
E 2
Total years at risk
X = case
17
x
x
ID = 2/17 = 11.8/100 person-years of observation

6. Bath tub- the relationship between Incidence and
Prevalence
Prevalence;
Existing cases in
the population
IN
Incidence;
Flow from healthy
to diseased state
OUT
Flow from diseased
state;
Recovered, died
v =Inc * Duration

7. Measures of Association
HIV risk among IVDU
entering NY Prison System
HIV+ HIV- Total
IVDU+ 61 75 136
IVDU- 27 312 339
Total 88 387 475
Risk in IVDU+=61/136=R1
Odds in IVDU+=61/75=O1
Risk in IVDU-=27/339=R2
Odds in IVDU-=27/312=O2
Relative Risk = R1/R2
Odds Ratio=O1/O2
Abs Risk difference is R1-R2
RR=a/a+b//c/c+d
OR=ad/bc

8. Rare disease assumption
If the outcome is rare (<5%) then the RR and
the OR are approximately the same.
In Cohort studies can use OR or RR
In Case Control Studies can only use OR.

9. Q1
Which of the following is/ are correct?
When you want to know the rate at which a population fall ill
from a disease you can use:
A: Prevalence
B: Cumulative Incidence
C: Incidence Rate

10. Q2
True or False?
The value for a relative risk is 1 - results below 1 indicates a
protective effect of the exposure and results above 1 is
evidence of a harmful effect of the exposure.

11. Q3
Cases Controls
Exposed A B
Not exposed C D
True or False?
The odds ratio is calculated as follows:
(A/B) / (C/D) = AD / BC
The risk ratio is calculated as follows:
(A*B)/(C*D) = AB/CD
True
False (A/(A+B))/ (C/(C+D))

12. Q4
True or False?
When measuring Cumulative Incidence a time
factor is automatically built in and does not have
to be stated in the interpretation.
Q5
• Approximately 5 million women live in Sweden.
• The incidence rate of breast cancer among these
women is 100/ 100,000 person-years.
• How many women get breast cancer every year
in Sweden?

13. Q7
To report the incidence rate of disease x, it would
be necessary to know.
a) Age distribution of the population
b) Case fatality rate
c) Mid year population at risk
To report the prevalence of the disease X it would
be necessary to know
a) Number of cases at a given time.
b) Duration of illness.