1. Basic Measurements in Epidemiology
Dr. Tauseef Ismail
Assistant Professor
Dept of C Med KGMC

2. Learning objectives Students will be able to
Understand the different tools of measurements in Epidemiology.
Explain why rates are important in Epidemiology and list some of the commonly used rates.
Define incidence and prevalence rates and provide examples of each.
Apply them to answer Public health problems

3. Epidemiology focuses on measurement of mortality & morbidity in human populations. The first requirement is therefore definition of what is to be measured & establishment of criteria or standards by which it can be measured.
This is not only a prerequisite of epidemiological studies, but also one of its goals.
Clear definitions help to minimize errors in classification of data.
Standardized methods of observation & recording are therefore essential before commencing any epidemiological study.

4. The scope of measurements in Epidemiology is very broad & includes the following:
Measurements of mortality, morbidity, disability, natality (birth rate).
Measurements of the presence, absence or distribution of the characteristics or attributes of the disease.
Measurements of medical needs, health care facilities, utilization of health services etc.
Measurement of the presence, absence or distribution of the environmental & other factors suspected of causing the disease.
Measurement of demographic variables etc.

5. Basic Tools of Measurement in Epidemiology are:
Rates
Ratios, &
Proportions

6. Rate: when we say that there were 500 deaths from motor vehicle accidents in city A during 2014, its nothing more than counting deaths in that city during that particular year.
It conveys no meaning to an epidemiologist who is interested in comparing the frequency of accidents in city A with that in city B.
To allow such comparisons, the frequency must be expressed as a rate. It is the basic measure of disease occurrence

7. Rate: = No. of events in a specified period x K
Pop. at risk in a specified period
A rate comprises a numerator, denominator, time specification & multiplier. The time dimension is usually a calendar year. Rate is expressed per 1000, 10,000 or 100,000 selected according to convenience to avoid fractions
Rate is used to estimate probability or risk of occurrence of a disease or to assess the accessibility or coverage of healthcare system.
Example
Crude death rate= Number of deaths in one year X1000
Mid – year Population

8. 2. Ratio:
Relationship b/w 2 numbers expressed as
x:y or x/y e.g ratio of males to females 2:3.
The numerator is not a component of the denominator.
E.g. Ratio of WBCs to RBCs is 1:600 or 1/600
Other examples include: doctor-population ratio, child-woman ratio, etc.

9. Proportion: Boys / Boys+ Girls= 1000 x 100 = 55% 1000+800
Specific type of ratio in which numerator is included in the denominator and the resultant value is expressed as %age. E.g 1: If there are 1000 boys and 800 girls in a school, the proportion of boys:
Boys / Boys+ Girls= x 100 = 55%
E.g 2: From 7,999 females aged 16 – 45 y, 2,496 use modern contraceptive methods.
The proportion of those who use modern contraceptive methods = 2,496 / 7,999 x 100 = 31.2%

10. All proportions are ratios, but not all ratios are proportions.
True
False
A. All fractions, including proportions, are ratios. But only ratios in which the numerator is included in the denominator is a proportion.

11. Attack Rate Defined as:
# of people at risk in whom a certain illness develops
total # of people at risk
Useful for comparing the risk of disease in groups with different exposures. The attack rate can be specific for a given exposure. For e.g. the AR in people who ate a certain food is called a food-specific attack rate.
# of people who ate a certain food & became ill
total # of people who ate that food

12. A person who acquires the disease from that exposure (from a contaminated food) is called a primary case.
A person who acquires the disease from exposure to a primary case is called a secondary case.
The secondary attack rate is therefore defined as the attack rate in susceptible people who have been exposed to a primary case.
It is a good measure of person-to-person spread of disease after the disease has been introduced into a population.
We often calculate the secondary attack rate in family members of the index case.

13. Secondary attack rate= No. of cases among contacts
of primary cases X 10n
total No. of contacts
Quiz: Of 75 persons who attended a church picnic, 46 subsequently  developed gastroenteritis. The attack rate of gastroenteritis is: 

14. 46/75 *100= 61%

15. Measurements of morbidity
Incidence : The no. of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease.
Incidence= No. of new cases of a specific
disease during a given time period X1000
pop. at risk during that period
The choice of 1000 is completely arbitrary– we could have used 10,000, 1 million or any other figure.

16. E.g. of incidence:
if there had been 500 new cases of an illness in a population of 30,000 in a year, the incidence would be:
=500/30,000 x 1000
=16.7per 1000 per year.
NOTE: the incidence rate must include the unit of time used in the final expression.

17. 2. Prevalence: is the number of affected persons present in the population at a specific time divided by the number of persons in the population at that time:
Measures the burden of disease in a pop. including old & new cases
Two types:
Point P (day, week, month)
Period P
Point P is the commonly used term

18. Point prevalence is the number of all current cases (old & new) of a disease at one point in time in relation to a defined population. The “point” in point prevalence may consist of a day, several days or even a few weeks depending upon the time it takes to examine the population sample.
Point P = # of all current cases (old & new) of a specified
disease existing at a given point in time X 100
estimated population at the same point in time

19. Period Prevalence
Is the proportion of a population that has the characteristic or disease at any point during a given time period of interest. “Past 12 months” is a commonly used timeframe. It includes cases arising before but extending into or through to the year as well as those cases arising during the year.
Period P= # of existing cases (old & new) of a specified
disease during a given period of time interval X 10n
estimated mid-interval population at risk

20. Point prevalence refers to the prevalence measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date.
Period prevalence refers to prevalence measured over an interval of time. It is the proportion of persons with a particular disease or attribute at any time during the interval.

21. Relationship b/w Incidence & Prevalence:
Prevalence depends upon 2 factors, the Incidence & Duration of illness. Assuming the pop. to be stable, & I & D are unchanging, the relationship b/w I & P can be expressed as:
P = I X D (incidence x mean duration)
Example (for a stable condition)
Incidence = 10 cases/1000 pop./year
Mean duration of disease = 5 years
Prevalence = 10 x 5= 50/1000 population
Conversely
I = P/D D = P/I

22. Relationship b/w Incidence & Prevalence

23. Relationship b/w Incidence & Prevalence
2012
2013
2014

24. Prevalence is Increased by
Longer duration of disease
Prolongation of life without cure
Increase of new cases (i.e incidence)
In migration of cases
Out migration of healthy people
In migration of susceptible people
Improved diagnostic facilities

25. Prevalence is Decreased by
Shorter disease duration
High case fatality from disease
Decrease in new cases
In-migration of healthy people
Out-migration of cases or susceptible people
Improved cure rate of cases

26. Use the following choices for the characteristics or features listed below:
Incidence
Prevalence
____ Measure of risk
____ Generally preferred for chronic diseases without clear date of onset
____ Used in calculation of risk ratio
____ Affected by duration of illness

27. Key:
1. A. Measure of risk 2. B. Generally preferred for chronic diseases without clear date of onset 3. A. Used in calculation of risk ratio 4. B. Affected by duration of illness
Incidence reflects new cases only; incidence proportion is a measure of risk. A risk ratio is simply the ratio of two incidence proportions. Prevalence reflects existing cases at a given point or period of time, so one does not need to know the date of onset. Prevalence is influenced by both incidence and duration of disease — the more cases that occur and the longer the disease lasts, the greater the prevalence at any given time.

28. Measurements of mortality
Crude Death rate:
CDR= No. of deaths occurring in a specified 12 months period X 1000
No of persons in the pop. at the mid-point
of the 12 month period (mid year pop)
Cause Specific Death Rate:
CSDR = No of deaths from a specific
cause during a calendar year X 1000
No of persons in the mid point
of that period

29. Age Specific Death Rate: ASDR = No. of deaths of a specific age group
No of persons in the pop. of that age
IMR is a special age specific death rate
Formula: No of infants dying during 1 year x1000
No of live births during the same period
IMR is an important indicator of the general health conditions of a pop.
Quality of life indicators: IMR, Literacy rate and GNP

30. Measurements of mortality
Case Fatality Rate (ratio) =
Total # of deaths due to a particular disease X 100
Total # of cases due to the same disease.
It is ratio of deaths to cases.
It is mainly used in Ac. Infectious diseases – food poisoning, cholera, measles.
It is the killing power of a disease and is closely related to virulence.
The CFR for the same disease may vary in different epidemics because of changes in the agent, host & environmental factors

31. Proportional Mortality Rate
It is sometimes useful to know what proportion of total deaths are due to a particular cause (e.g., cancer) or what proportion of deaths are occurring in a particular age group (e.g., above the age of 50 years).
It expresses the “ # of deaths due to a particular cause (or in a specific age gp.) per 100 (or 1000) total deaths.”

32. Examples of Proportional Mortality Rate;
Proportional mortality from a specific disease =
# of deaths from a specific disease in a year x 100
total deaths from all causes in that year
b) Under – 5 proportionate mortality rate=
# of deaths under 5 years of age in the given year x 100
total # of deaths during the same period
Proportional mortality rate for aged 50 yrs & above =
# of deaths of persons aged 50 yrs & above x 100
total deaths of all age groups in that year

33. Proportional Mortality Rate

34. Assignment: Indicators of Health (to be submitted by the CR on 29/11/2017 in the faculty office)
Mortality indicators
Morbidity indicators
Disability rates
Nutritional status indicators
Health care delivery indicators
Utilization rates
Indicators of social and mental health
Environmental indicators
Socio-economic indicators
Health policy indicators
Indicators of quality of life

35. References
Gordis L. Epidemiology. 5th Edition. Canada: Elsevier Saunders; 2014.
Aschengrau A, Seage III GR. Essentials of Epidemiology in Public Health. 2nd Edition. USA: Jones and Bartlett Publishers; 2008.
Park K. Preventive and Social Medicine. 20th Edition. India: M/s Banarsidas Bhanot Publishers; 2009.