Epidemiology

1. Principles of Epidemiology
and Disease Surveillance
Dr. Tubosun A. Olowolafe
Department of Public Health
Lead City University, Ibadan

2. Measures of Disease Frequency
• The measure of health and disease is fundamental to the practice of
epidemiology
• Measures of disease frequency come in five main flavors
• Counts
• Proportions
• Percentages
• Ratios
• Rates

3. Proportions
• Proportion = No. of people who have attribute Z
• No. of people in the population
• For proportions, the numerator is included in the denominator.
• Percentage = proportion x 100 = %

4. Ratio
• Ratio = No. of events of type x
• No. of events of type y
• = x/y
• For a ratio, the numerator is not included in the denominator.

5. Rate
• Change in one quantity divided by the change in another
quantity in a defined population over a defined period of time

6. Expressing Measures of Disease Frequency
• Any epidemiological measure can be expressed as
• m per 100 or
• m per 1,000 or
• m per 10,000 or
• m per 100,000 or etc.
• The choice of the multiplier is arbitrary.

7. Basic measurements
measure Ratios Proportions Rates
Morbidity Risk ratio, rate ratio, odds ratio Attributable proportion,
Point prevalence
Incidence rate, attack rate,
secondary attack rate
Mortality Death to case ratio), Maternal
mortality rate
case fatality rate, Crude mortality rate
Cause specific mortality rate,
Age and sex specific mortality
rate Perinatal, Neonatal and
infant mortality rate

8. Measures of Morbidity
• Prevalence Rates
• Point Prevalence Rate
• Num: # of current cases of a specified disease at a given point in time
• Den: estimated population at the same point in time
• Expressed per number at risk (10n )where n =2,3,4,5,6
• Period prevalence Rate
• Num:# of current cases of a specified disease identified over a given time interval
• Den: estimated population at mid interval
• Expressed per number at risk (10n), n=2,3…..6

9. Measures
• Incidence rate ( Cumulative)
Num: # of new cases of a specified disease reported during a given time interval
Den: average population during time interval
Expressed per number at risk 10n where n = 2,3,4..6
• Incidence Density
Num: # of cases during observation period
Den: Time each person was observed totaled for all person
Expressed per person-time of observation 10n
• Attack rate
Num:# new cases of a specified disease reported during an epidemic period
Den: population at start of the epidemic period
expressed per number at risk 10n where n =2
• Secondary attack rate
Num:# new cases of a specified disease among contacts of known cases.
Den: Size of contact population at risk
Expressed per no at risk 10n where n=2
• Duration of a disease

10. Cumulative Incidence
•is . . .
• One of the most widely used measures of
disease risk.
• = No. of new cases during specified time period
• No. of people at risk at the beginning of the time period

11. Example of cumulative incidence
• In a screening of 1,000 men, 100 were found initially to have
CVD. During the 10-year period another 200 men developed
CVD.
• Cumulative incidence
• = 200 new cases of CVD during the 10-year follow-up
900 people in the population at risk
= 22%
• Interpretation: During the 10-year follow-up, the risk of
developing disease is 22% in this population.

12. Incidence Rates
• For individual data:
• Rate = No. of events (x)
person-years (t1 + t2 + . . .+ tN)
• For large population data:
• Rate = No. of events (x)
person-years (ave. pop size * duration)
• where duration is the length of the study
• Note: For a rate, the numerator is not included in the
denominator. The denominator is a different dimension
altogether.

13. Person - Time
• Person Person-months
1 Disease 10
2 13
3 Loss to follow-up 9
4 15
5 Disease 10
6 15
7 Disease 7hs 1 2 3

14. Example of incidence rate using individual
data
• Incidence Rate using individual data
= No. of events (x)
person-years (t1 + t2 + . . .+ tN)
3 new cases of disease
10 + 13 + 9 + 15 + 10 + 15 + 7 months of observation
= 3 new cases of disease
79 person-months
• Interpretation: The rate of developing disease was 3 per 79
person-months of observation in the population over time T

15. • Incidence Rate = No. of events (x)
Incidence Rate = No. of events (x)
person-years (avg. pop size * duration
person-years (avg. pop size * duration)
• = 200 events/ 900 people*10 years
Example of incidence rate using large
population data

16. Relationship between Prevalence and
Incidence
• Incidence is a proxy for “risk” whereas prevalence is best for
assessing disease burden or case load in a geographic area.
• There is a well known relationship between them, namely -
Prevalence = Incidence x Duration of disease
P = I x D

17. Examples of P=I x D
• If the incidence of diabetes mellitus is 1% per year and its
approximate duration is 5 years, then what is its expected point
prevalence?
• 2. Assuming equal incidence of disease, which is more
prevalent, pancreatic cancer or brain cancer?
Average duration of Pancreatic Cancer = 3 months
Average duration of Brain Cancer = 1.5 years