Lecturer: Professor Gary J Macfarlane

1
Introduction to Epidemiology Measurement of
Disease Occurrence

Lecturer Professor Gary J
Macfarlane Course, Module BSc,
Epidemiology/Statistics Module Date
2nd October 2007

2
Epidemiology
The study of the occurrence, distribution and
determinants of disease (and other
health-related conditions) in populations
3
Objectives
To determine the magnitude and impact of
diseases (or other conditions) in populations
1.
4
Chief causes of death in the United States, 1900
and 1982
1900
1982
Pneumonia/influenza
11.8
Heart disease
34.4
11.2
Cancer
23.9
Tuberculosis
Heart disease
9.4
Accidents
6.6
7.6
Stroke
6.5
Stroke
6.3
Chronic lung disease
2.9
Diarrhoea/enteritis
5.9
Suicide
2.1
Nephritis
4.5
Pneumonia/influenza
2.0
Cancer
4.2
Chronic liver disease
1.9
Accidents
1.9
Diabetes mellitus
1.7
Diptheria
37.2
Other
18.0
Other
Source U.S. D.H.H.S., Prevention 84/85.
Washington, D.C. Public Health Service Office,
1985
5
Objectives
2.
Surveillance programmes - collecting information
on disease frequency in populations to
- evaluate the efficacy of therapeutic
measures - detect unexpected changes in disease
occurrence
6
Surveillance Programmes
Cancer
7
Trends in Testicular CancerIncidence and
Mortality
Age-standardised Rate per 100000 person-years
8
Surveillance Programmes
Cancer
Infectious disease
Creutzfeldt-Jakob Disease
9
Surveillance Programmes
Cancer
Creutzfeldt-Jakob Disease
10
Surveillance of CJD
Number of New cases
Year
11
Objectives
3.
To determine the natural history and clinical
course of diseases
12
Objectives
4.
To identify causes of disease
Observational Epidemiology
13
Death rates from cholera, 1853-1854 according to
water company supplying subdistrict of London
Cholera deaths in 1853-1854
Population in 1851
Deaths per 100,000 living
Water company
Southwark and Vauxhall
167,654
192
114
Both companies
301,149
182
60
Lambeth
14,632
0
0
Snow (1855)
14
Death rates from cholera, 1853-1854 according to
water company supplying actual house
Number of houses
Deaths from cholera
Deaths per 10,000 living
Water company
Southwark and Vauxhall
40,046
1263
315
Lambeth
26,107
182
37
Rest of London
256,423
1422
59
Snow (1855)
15
Measures of Disease Occurrence
Rates
Prevalence
Incidence
16
Prevalence (rate/proportion)
17
Prevalence
Point Prevalence
Period Prevalence
18
Point prevalence
19
Period Prevalence
Diseases vary within individuals
Have disease but not in state
e.g. asthma
20
Example
In Littletown Grammar School, which has 4700
pupils in total,12 were recorded as having an
upper respiratory tract infection (URTI) on
1/6/2002 while 53 were recorded as having an URTI
at least some time during the month of May 2002.
Qu Point prevalence on 1/6/2002 ?
Qu Period prevalence during May 2002?
21
Incidence rate
Is the number of new disease onsets per
specified unit of population during a specified
period of time
22
Example
Among a cohort of 200 men aged 35-64 years
register in May 2001 with the General Practice in
Hilltop village and who initially were free of
back pain, 10 develop a new episode of low back
pain during the following year
What was the incidence rate of new back pain
episodes in the one year May 2001-2002?
23
Example
24
Incidence Rate
Incidence rate disease onsets (n)
? time periods at risk
The denominator is referred to as a measure of
person-time and refers to the time for which a
person is at risk of the disease under
consideration
25
Cumulative Risk
Cumulative Incidence records all onsets over a
certain period of time e.g. from 0-64
years from 35-64 years over lifetime
26
Relationship between incidence and prevalence
Prevalence ?
Duration
X
Incidence
Steady State Prevalence Incidence x Duration
27
Reprise
In what circumstances may incidence be the most
appropriate measure of disease occurrence?
28
Reprise
In what circumstances may prevalence be the most
appropriate measure of disease occurrence ?
When would
a) Point prevalence
b) Period prevalence be preferred
29
Routine sources of morbidity and other statistics
Birth statistics
Mortality statistics
Morbidity statistics
Communicable disease
General practice morbidity statistics
Cancer incidence
30
National Health Service Central Register
Includes all persons with a general practitioner
Records Identifying information
Name
Date of Birth
NHS Number
Vital Status
Incident Cancers
Local Health Authority