1. in descriptive studies
Measures
in descriptive studies
Lecture on measures of disease occurence
Alicia Barrasa

2. Descriptive studies - context in epidemiology
Epidemiology studies the occurrence of disease:
the frequency and distribution of diseases and their determinants in the population
frequency and distribution: who, when, where
determinants: why

3. Descriptive studies - context in epidemiology
where
who
when
why
hypothesis
descriptive
analytical
population
individual
observational
experimental
Clinical trials
ecological
cross-seccional
cohort
case-control

4. Descriptive studies - context in epidemiology
Measuring disease occurrence
Descriptive
identification of disease determinants
Analytical
Use:
Analysing surveillance data
Outbreak investigation
Aim:
Response / intervention

5. Descriptive studies – measuring disease occurrence
counts
number of cases
“we have 5 cases of tetanus”
On its own it is very little informative!!
Simple / most frequently performed measure in epidemiology
Refers to the number of cases of a disease e.g. (or other health phenomenon)
Limited usefulness for epidemiologic purposes without knowing size of the source population
Who is in the denominator ????
In what time period did they occur???

6. Proportion Ratio Rate Measuring disease occurrence
What, who is in the denominator ????
In what time period did they occur???

7. Proportion 400 / 1000 40% = The division of 2 numbers
Numerator INCLUDED in the denominator
In general, quantities are of same nature
In general, ranges between 0 and 1
Percentage = proportion x 100
males
population =
400 / 1000
40%

8. Ratio
The division of two numbers
Numerator NOT INCLUDED in the denominator
Allows to compare quantities of different nature
males
females =
5 / 2
2.5 /1
beds
doctors
850 / 10 =
85 / 1
3 / 1 =
participants
facilitators

9. Rate
The division of two numbers
TIME INCLUDED in the denominator
Speed of occurrence of an event over time
Births in 2007
Population in 2007 =
2000 /
= = 1.3 per 10,000 inhabitants per year
Rate may be expressed in any power of 10:
100, 1000, 10000, …

10. Number of cases of disease
Measuring disease occurrence
Number of cases of disease
Population
Number of cases of a disease in a given population at a specific time
Proportion of the population that had the disease at a given time
Probability of having the disease
una enfermedad muy frecuente pero de rápida evolución hacia la curación o hacia la muerte puede ser de baja prevalencia, ya que muchos de los casos no los veremos al no coincidir con el momento en que “miremos” la población.
una enfermedad de larga duración se irá acumulando en estado de enfermedad y será detectada cuando demos un “corte transversal” a la población.
prevalence

11. incidence Measuring disease occurrence (cumulative incidence)
Number of NEW cases of disease during a period
Population at the beginning of the period
Number of new cases of a disease in a given population at a specific time
Proportion of the population that acquires or develops a disease in a period of time
Probability of developing a disease
The cumulative incidence is often also called the incidence proportion
In our numerator we will have the number of new cases of disease during a specified period
And in the denominator the population at the beginning (AT RISK) of the observation period
incidence
(cumulative incidence)

12. Number of NEW cases of disease Total person-time of observation
Measuring disease occurrence
Incidence Rate
Proportion of the population that acquires or develops a disease in a period of time
Speed of developing a disease
Number of NEW cases of disease
Total person-time of observation
It is more precise because it includes the time at risk in the denominator. The incidence rate is the number of new cases divided by the total number of person time observed
Denominator:
- is a measure of time
- the sum of each individual’s time at risk and free from disease

13. Cumulative Incidence = 3 cases / 6 persons = 50%
Time-person
Person 1 l 3
Person 2 l x 4
Person 3 l 6
Person 4 l x 3
Person 5 x l 1
Person 5 l 5
22 p.y
2003
2004
2005
2006
2007
2008
2009
Cumulative Incidence = 3 cases / 6 persons = 50%
Incidence Rate = 3 cases / 22 person-years = 0.14
= 14 cases / 100 person-years

14. Outbreak of cholera in country X in March 1999
Measuring disease occurrence
Attack Rate
Cumulative incidence during an outbreak
Expressed for the entire epidemic period, from the
first to the last case
Not really a rate but a proportion!
Outbreak of cholera in country X in March 1999
Number of cases
Population 18,600
Attack rate %
The attack rate is the cumulative incidence during a outbreak, expressed for the entire epidemic period from the beginning to the end of the epidemic
It is imprtant to know that it is rather a proportion than a rate
What is special about a proportion?
Persons in the numerator are always included in the denominator. It indicated the magnitude of a part, related to the total.
In epidemiology, it tells us the fraction of the population that is affected. A/A+B
The numerical value of a proportion lies from 0 to 1, if multiplied by 100, you get the percentage

15. Measuring disease occurrence developing the disease
Descriptive
Prevalence
Incidence
Probability of
having the disease
Probability of
developing the disease
Risk: Non-technical definition
Vague, culture-dependent
Unexpected, unusual, dangerous/negative events
Risk: Epidemiologic definition
Probability that an event will occur
Estimated by:
observing events among a population
during a specified time
Burden
RISK

16. Measuring disease occurrence
Let’s talk about ODDS now
Probability that an event will happen
Probability that an event will not happen
Won
Lost
Total
EPIET facilitator team 14 1 15
Odds, is a special measure of disease / event occurence.
The odds are often used in horse races and are a ratio
The odds are the probability that an event will happen divided the probability that an event will not happen
Is a ratio
Lets make an example and look at the number of football matches which have been won by facilitators between 1995 and 2004
The odds of facilitators winning the soccer match= the prob that they will win divided probability that they will loose.
Facilitaors have won all but one of the 15 matches
14 / 15
Odds of winning = = 14:1 = 14
1 / 15

17. Risks, Odds and 2x2 tables Cases Non cases Exposed a b a+b Non exposed
c+d
a+c
b+d
Risk of being a case in exposed = a / (a+b)
Risk of being a case in non exposed = c / (c+d)
Odds of exposure among cases = (a/(a+c))/(c/(a+c))= a/c
Odds of exposure among non cases = (b/(b+d))/d/(b+d))= b/d

18. Summary of disease occurence
PREVALENCE
INCIDENCE
Cumulative incidence
Incidence rate
Attack rate
ODDS
Summarising the measurements of disease occurence
We will have the risk
Expressed as cumulative incidence or incidence proportion
And in outbreak situations the attack rate which is a proportion but not a rate
A rate includes a measurement of time in the denominator and is expressed as incidence rate
And the odds is a ratio of the probabilty that an event will occur divided by the probability that an event will not occur… it is a measurement of risk for rare events
It is limited between 0 and 1…. Meaning between 0 and 100%
The attack rate is used in outbreaks but is a proportion and NOT a rate, because the time is not included in the denominator

19. examples Cases of diabetes in 2008 in Spain Spanish population in 2008
30 guests with diarrhoea
300 wedding guests
80 hours at the Lazareto bar
150 hours in the lecture room
Number of deaths in 2007 in Spain
Spanish population in 2007
Proportion
Attack rate (proportion)
Ratio of time spent in the bar and time spent in the lecture room
Mortality rate

20. References Epidemiology, an introduction. Rothman KJ.
Epidemiology in Medicine. Henneckens CH, Buring JE.
Modern infectious disease epidemiology. Giesecke J.
Dictionary of epidemiology. Last J.