1. Measures of disease occurrence
Tunis, 28th October 2014
Acknowledgments: T Ancelle , A Bosman, D Coulombier, A Moren, P Sudre, M Valenciano, J Fitzner, S Hahné, P Penttinen, P Kreidl, B Schimmer
Dr Eugena Tomini
Institute of Public Health - Tirana, Albania

2. Epidemiology Descriptive epidemiology measuring disease occurrence
Analytical epidemiology
identification of disease determinants
Use
Analysing surveillance data
Outbreak investigation
Aim
Response / intervention
When analysing surveillance data or when you do an outbreak investigation, one generally uses two kinds of epidemiology: first
Descriptive epidemiology in order to measure disease occurence,
And analytical epidemiology to identify disease determinants.
These measures can aid in quantifying potentially causal relations between exposure and disease,
Based on this information, an effective response or intervention can be organised..

3. Epidemiologic approaches
DESCRIPTIVE
Health and disease in the community
What? Who? When? Where?
What are the
health problems
of the
community?
attributes of
these illnesses?
How many people
are affected?
What are the
attributes of
affected persons?
Over what
period of time?
Where do the
affected people
live, work or
spend leisure
time?
Each of the measures can be calculated for different combinations of “What? Who? When? and Where?”
Each of the W’s needs to be defined carefully to get comparable measures across state, nation, world

4. “we have 5 cases of tetanus”
Counts
Number of cases
“we have 5 cases of tetanus”
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
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
On its own very little informative!!
What, who is in the denominator ????
In what time period did they occur???

5. Descriptive Epidemiology
Ratio
Proportion
Rate
In this presentation I will discuss several measures of disease frequency such as risk, cumulative incidence, attack rates and prevalence. But first we have to understand how a ratio, proportion and a rate are defined in terms of what and who is contained in the denominator.
What, who is in the denominator ????
In what time period did they occur???

6. Ratio = 5 / 2 = 2.5 / 1 The division of 2 numbers
Numerator NOT INCLUDED in the denominator
Allows to compare quantities of different nature
Occurence of disease, think about numerator and denominator.
Let’s start with a ratio:
A ratio is a quotient of two numbers. The numerator does not include in the denominator. It allows to compare quantities of different nature.
= 5 / 2 = 2.5 / 1

7. Ratio, Examples # beds per doctor 850 beds/10 doctors
R = 85 beds for 1 doctor
# participants per facilitator
3 participants for 1 facilitator
Sex ratio: Male / Female Female / Male
Odds ratio, Rate ratio, Prevalence ratio
For example number of beds per doctor, or the number of participants per facilitator, or number of inhabitants per latrine. Another example is the male to female ratio or the female to male ratio. Other examples are odds ratio, rate ratio or prevalence ratio.

8. Proportion 5 3 --- = 0.6 = 60% The division of 2 numbers
Numerator always included in the denominator
Quantities have to be of same nature
Proportion always ranges between 0 and 1
Percentage = proportion x 100
A proportion is a special ratio, also a quotient of 2 numbers. The numerator is always included in the denominator. Also the quantities have ot be of the same nature. It always ranges between 0 and 1. The percentage is calculated from the proportion multiplied by 100. For example 3 out of 5 persons are female. So the proportion of females in this group is 60% 3
--- = 0.6 = 60% 5

9. Proportion, Examples
Proportion of injecting drug users (IDU) among cases of tetanus in the UK
141 cases of tetanus, 2 IDU
Percentage of IDUs among cases of tetanus is 2/141 = = 1.4%
Another examples of a proportion is the number of intravenous drug users among 141 cases of tetanus (during an outbreak of tetanus in the UK). The proportion of intravenous drig users among the group of tetanus cases is 2/ 141= 0.014, which corresponds to 1.4%

10. Rate 100 2 ----- = 0.02 / year The division of 2 numbers
Speed of occurrence of an event over time
Numerator
number EVENTS observed for a given time
Denominator
POPULATION (+ observation time)
Another expression, also division of two numbers
A rate is a special kind of ratio: quotient of two numbers,
Add the speed of occurence of an event over time
Includes the time the population that the persons are observed.
Observed in 2008 2
----- = 0.02 / year
100

11. Example: rate Mortality rate of tetanus in France in 1995
Tetanus deaths: 17
Population in 1995: 58 million
Mortality rate 17/58x106 = 0.29 per 1,000,000 per year
Rate may be expressed in any power of 10
100; 1,000; 10,000; 100,000; 1,000,000

12. Measures of disease occurence
Epidemiology :
« Study of the occurrence of illness »
Types of measures :
Prevalence
Incidence rate (incidence density, cumulative incidence, attack Rate)

13. Risk
What does this have to do with epidemiology, as we generally talk about risks

14. Risk Non-technical definition Epidemiologic definition
Vague, culture-dependent
Unexpected, unusual, dangerous/negative events
Epidemiologic definition
Probability that an event will occur
Estimated by:
Observing events among a population
during a specified time
Commonly used the term risk can mean many things, is usually used for negative events quite vague and may depend very much on culture
As epidemiology is a precise science there is a clear definition for risk
Which is the probability that an event will occur
By observing events in a population during a specified time

15. Cumulative Incidence (CI) Incidence Proportion
Number of new cases of disease during a period
Population at the beginning of the period
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 of the observation period

16. Cumulative Incidence Incidence Proportion (ii)
Number of new cases of disease during a period
Population at the beginning of the period
Ex: Echinococcosis in country ‘X’ in 1979:
New cases ,250
Population 350,000
Cumulative incidence = = 0.36 %
= 3.6 new cases / 1000 persons
In an example of echinococcosis in country“X“ in during 1979 a total of 1250 new cases were observed in a population of 350,000 inhabitants which results in a cumulative incidence of per year. This can also be expressed as 0,36% of the population newly acquired the diseased during 1979
Number (difficult to understand this number (small number), 3.6 new cases/ 1000 during a year.It is important to express this number in the best way.
It can also be expressed as 3.6 new cases per 1000 inhabitants during the period of the year 1979

17. Cumulative Incidence Population = 12 Diseased = 7 x x disease onset
Month 1
Month12
Risk of acquiring the disease in a given population in a given period of time
Lets make a graphical example
Lets assume that in
a population of 12 persons
7 get a certain disease
Population = 12 Diseased = 7

18. Cumulative Incidence (ii)x
Month 1
Month12
CI = 7/12
= 0.58
= 58%
Any person living in this population will have a risk of 58%.
The cumulative Incidence or incidence proportion is the % of population that get a disease in certain period of time
In our case 7 of 12 persons which is 58% will get the disease during the year of observation

19. Cumulative Incidence (iii)x
Month 1
Month12
CI = 7/12
= 0.58
= 58%
Cumulative incidence or incidence proportion assumes that the entire population at risk is followed for the same period of time
CI assumes that the entire population at risk
is followed up for the same time period

20. Attack Rate (AR) 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!
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
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

21. Attack Rate (AR) Number of cases 490 Population 18,600
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!
Ex: Outbreak of cholera in country X in March 1999
Number of cases
Population 18,600
Attack rate %
It is a cumulative incidence withing an outbreak. Attack rate is used widely everywhere.
During an outbreak of cholera in country X in 1999 among a population of 18, persons got the disease
Resulting in an attack rate of 2,6%

22. Rate
What does this have to do with epidemiology, as we generally talk about risks

23. Incidence rate Number of new cases of disease
Total person – time of observation
Rate
Denominator:
- is a measure of time
- the sum of each individual’s time at risk and free from disease
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
It is improtant to know, when using the term rate - a measurement of time is included in the denominator and
That only persons are included in the denominator until they either develop disease or they are not at risk for developing disease any more (i.e diseased, moved out, immune, lost, …)
So the denominator is the sum of each individuals time at risk and free from disease
Lets say what it means on a graph

24. Person-time x x A B C D E 6.0 10.0 8.5 5.0 Total years at risk 35.5
Time at risk A B C D E
6.0
10.0
8.5
5.0 x x
Assume a population of 5 persons
Person A is followed from 1990 until 1996 … because he may die or be lost to follow up… he will contribute 6 years to our observation
Person B is born in 1992 and develops disease in He or she will contribute 6 years to our observation
Person C contributes to the entire observation period of 10 years
Person D moves to the study area in 1991 and leaves the area in mid He will contribute for 8,5 yrs
Person E develops disease after 5 yrs
So our denominator will be the sum of the time of each individualat risk when they are free of disease which in our example are 35.5 years
Total years at risk
-- time followed
x disease onset

25. Incidence rate (IR) (Incidence density)
Time at risk
IR = 2 / person years
= cases / person year
= 5.6 cases / 100 person years
= 56 cases / 1000 person years A B C D E
6.0
10.0
8.5
5.0 x x
This total person time observed will become our denominator and the number of persons who diseased are in the numerator
Therefore the incidence rate or incidence density will be 2/35,5 person years which equals cases per person year
Which is the same as 5.6 cases per 100 person years
Or expressed as 56 cases per 1000 person years
Note that the incidence density is not limited and may reach infinite values
Total years at risk
-- time followed
x disease onset

26. Prevalence status of disease
What does this have to do with epidemiology, as we generally talk about risks

27. Prevalence (point prevalence)
Number of cases of disease at a specific time
Population of interest at that time
Prevalence is a proportion – range of 0 to 1
Removes the effect of total population size – makes estimates from different populations or over time more comparable

28. Prevalence (point prevalence)(ii)
Number of cases of disease at a specific time
Population of interest at that time
Proportion of a population affected by a disease
at a given time.
Expressed as a percentage

29. Prevalence (point prevalence)(iii)
Number of cases of disease at a specific time
Population of interest at that time
Proportion of a population affected by a disease
at a given time.
Expressed as a percentage.
Ex: Echinococcosis in country ‘’Y’’ in 1980:
Cases 96,200
Population 350,000
Prevalence %

30. Relationship between Incidence, Prevalence and Disease Duration
Deaths,
Cured,
Lost...
Duration

31. Incidence and Prevalence
Incidence and prevalence measure different aspects of disease occurrence
Prevalence
Incidence
Numerator
All cases no matter how long diseased
Only NEW cases
Denominator
All person in population
Only person at risk of disease
Measures
Presence of disease
Rrisk of disease
Most useful
Resource allocation

32. Case fatality
Case fatality is the concept used to express the proportion of cases of a certain disease that actually dies due to the consequences of that disease. Since it is a proportion, it is usually expressed as %, or per 1000.
The case fatality can be seen as a cumulative incidence. It is relevant to keep in mind that the death has to be due to the consequences of the disease, since otherwise each disease would have a case fatality of 100% (since all people die eventually). It is a true proportion, since the denominator includes all cases, even those who died (the numerator).
Example of case fatality: around 1850, the case fatality of cholera (for which then there was no effective treatment) was up to 40%. This means that out of each 100 cases of cholera, 40 would eventually die due to the disease, usually within 2 weeks after onset. In comparison, the case fatality of tuberculosis in those times was almost 100% within the first 2 years after diagnosis, since there was no cure for tuberculosis either

33. Mortality rate
Number of deaths Mortality rate = –––––––––––––––––––––––––––– Population at risk × Time interval
Mortality = number of deaths per population (most often per )
Proportional mortality =  of all people who died, what proportion have died from this disease
If there are 25 lung cancer deaths in one year in a population of 30,000, = mortality rate for that population is 83 per 100,000.

34. Summary of disease occurence
Risk
Cumulative incidence or incidence proportion
Attack rate
Ratio
Rate
Incidence rate (Incidence density)
Prevalence
Mortality
Case fatality
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

35. What it is….? Number of deaths in 2007 in Spain
Spanish population in 2007
16 malaria deaths
1000 deaths occured
30 guests with diarrhoea
300 wedding guests
10 hours at the bar
15 hours in the lecture room
Mortality rate
Proportion
Attack rate (proportion)
Ratio of time spent in the bar and time spent in the lecture room

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

37. Thank you!
Dr Eugena Tomini