Epidemiology: Indicators and Measurement Tools

Indicators in Epidemiology

• The lecture covers indicators in epidemiology, a course given to first-year medical students as part of the Social Medicine and Public Health module

Introduction to Indicators

• Population health assessments typically involve indicator-based approaches
• Indicator-based approaches consist of collecting numerical data to highlight health needs and problems within a given population.
• Numerical data focuses on the determinants and states of health

Tools for Calculating Indicators

• Indicators are usually calculated using measurement tools
• Most measurement tools are fractions, which include a numerator and denominator
• Indicators are usually measured as ratios

Definitions of Measurement Tools

Proportion

• A proportion relates numerator and denominator and are measured simultaneously
• The result will be between 0 and 1 (or 0 to 100%)
• The numerator ("a") is included in the denominator ("b"), therefore being a part of it
• Represented as a/b x k, where k is a power of 10

Example

• In an sample of 600 persons includes 400 men and 200 woman
• The proportion of men in the sample is 400/(200+400) x 100 = 66.6%

Ratio

• A ratio relates the quantity of one group (numerator) to the quantity of another group (denominator)
• Groups belong to the same set but are mutually exclusive
• Expresses how much larger or smaller one quantity is relative to another
• An example is the "sex ratio" commonly used in epidemiology

Example

• In a sample there are 400 men and 200 women
• The men to women ratio is 400/200=2

Index (Rate)

• An index expresses the relationship between two mutually exclusive quantities with different numerators and denominators

Examples

• Number of hospital beds per inhabitant
• Number of dispensaries per inhabitant

Rate

• A rate considers the variable of time
• It measures the speed at which an event occurs over time
• It is expressed in terms of a unit of time in a given geographical location, and for a well-defined group of people
• A rate may measure disease, death, handicap, or relapse

Rate - Numerator and Denominator

• The numerator is the number of events occurring over a certain period
• The denominator provides the population exposed to the risk of the event occurring during that period
• The mortality rate in the French population is 8.5 per 1000 per year

Principal Health Indicators

Morbidity Measures

• Morbidity represents the number of persons suffering from a particular disease within a population
• Morbidity can be measured by incidence or prevalence

Incidence Measures

• Incidence is a frequency measure that quantifies new cases appearing in a given population during a specified period
• Incidence measures include:
• Incidence rate, which expresses how fast new cases appear
• Cumulative incidence, which represents the probability of an individual developing a disease over a period

Calculating Incidence rate

• Incidence rate has a formula of No. of new cases / Sum of persons-time at risk
• Incidence Rate requires the time each person is followed in the denominator, which is the total person-time at risk
• Person-time at risk is a unit combining the number of followed individuals and the duration of their follow-up
• The incidence rate on an acute disease epidemic is called the attack rate which uses a short incubation and duration period

Calculating Cumulative Incidence

• The formula represents the number of new cases over a period divided by the population initially at risk at the beginning of the period

Calculating Incidence - Interest

• Incidence is useful to epidemiologic surveillance as it helps measure the frequency variations of different diseases
• Incidence enables you to evaluate the impact of health programs and is an excellent guide to health action/planning

Diabetes Incidence Example

• In a series of 10 observations over 12 months:
• Person 1 had 6 months total follow up with a diabetes event ending due to the diagnosis being positive
• Person 2 had 4 months total with no events and the end date was due to being lost of sight
• Person 3 had 9 months total with no events and the end date was due to death
• Person 4 had 12 months total with no events and the end date was the end of the study period
• Person 5 had 8 months total follow up with a diabetes event ending due to the diagnosis being positive
• Person 6 had 5 months total with no events and the end date was due to being lost of sight
• Person 7 had 3 months total follow up with a diabetes event ending due to the diagnosis being positive
• Person 8 had 12 months total follow up with a diabetes event ending due to the diagnosis being positive
• Person 9 had 12 months total with no events and the end date was the end of the study period
• Person 10 had 6 months total follow up with a diabetes event ending due to the diagnosis being positive
• The people-time calculated is 6+4+9+12+8+5+3+12+7+6 = 72 months
• The rate is 4/72 x 1000 = 55.6 cases per 1000 person months
• These rates predict that a population would experience 55.6 new cases of diabetes per 1000 person months of follow-up
• If 100 people were followed for 10 months it presents a 1000 person-month sample
• At a rate of 55.6 cases per 1000 persons, it would be expected to see 56 new cases in a 10 month period

Acute Disease Incidence Example with the Flu

• A person contracting a chronic disease is excluded once contracted due to having no risk of reinfection
• A series of 10 observations were performed to evaluate flu incidence over a period of 30 days:
• Person 1 was followed all 30 days with no flu event and the end of the study
• Person 2 with 30 day follow up, contracted flu on J10 (day 10) so they took 5 day to recover and the adjusted followed up was 25 days
• Person 3 had 12 days follow up with now flu event and dropped out due to the person being lost to sight
• Person 4 contracted flu at day 5 and day 20, so was followed up for 20 days, the adjusted followed up was 20 days
• Person 5 had no flu event and were followed up all 30 days
• Person 6 had no flu event followed for 15 days and passing away
• Person 7 contracted flu at day 15 and then was followed up all 20 days without complication
• Person 8 had no flu event and were followed up all 30 days
• Person 9 had no flu event and were followed up all 18 days
• Person 10 contracted flu at day 12, but the effect of the flu convalescence was incomplete, so they were monitored for 12 days
• With a person time of 212 days and 5 event cases, the calculated incidence rate is 5/212 x 1000 = 23.6 cases per 1000 persons
• In a 1000 person-day observed period, close to 24 people would be predicted of exhibiting flue

Cumulative Incidence Example

• In an example of 1500 villagers in 2008, it was determined that 3 cases of tuberculosis had been diagnosed
• Cumulative Incidence of annual tuberculosis in the village was (3/1500)x100 =0.2 cases per 100 inhabitants

Prevalence Measures

• Prevalence is the proportion of subjects that already have illness out of the group that might contract the disease
• Prevalence is dependent on both the rate of disease and the time since it has been present

Types of Prevalence

• Instant Prevalence: Proportion tested that has disease out of time
• Period Prevalence: Proportion with disease who have not been tested

Instant Prevalence Formula

• Formula is Prevalence = Cases tested at a certain time/ Population tested at that time
• The measures the epidemic at a certain time that can enable action

Period Prevalence Formula

• Formula is Prevalence = Cases at the start of a period + New cases/ all people
• This provides an idea of a diseases range over a fixed time

Prevalence - Interest of Prevalence

• Provides a guide to chronic diseases so that resources can be allocated effectively

Prevalence - Example

• There is a village that has 1000 villagers
• Over a set period, there are 2 incidents plus 5 existing cases
• The prevalence is (2+5)/1000 which after multiplying by 100 equals 0.7%.

Measures of Mortality

• Considers instances of death in a population
• Mortality measures are divided into general and specific indicators

Rate of Mortality

• Rate shows deaths in a group to amount in the center of people
• Formula is deaths divided by time
• One in thousand deaths or %.

Rate example

• TBM in Morocco came up to 5.06 in 2019
• Every thousand would get a death in Morocco

Interpreting Rate

• Is not impacted by location
• Is not impacted by sex in the population
• Is not impacted by profession

Subsets for Specific Rate Study

• Age
• Sex (Mortality of women)
• Profession
• Environment (Urban / Rural)
• Marital state

Cause of Death and the Rate Formula

• = Amount due to cause/ Total middle class income and period
• Morality and cardiovascular come up to 2.17/ 1000

Infants that Die and Rate

• Infant morality is measured at 1 year age
• % and %.

Intrepretation

• Measured at 28.8 % in 2015
• Close to 28 out of 100 die before 1 years.

Calculation

• = cases * period / total in society
• Refers to reasons for fatal events

Interpretation

• Among one of 3 cancer, is fatal

Example

• 100 000 inhabitants
• Deaths 1200
• Cases of heart events is 400
• Cancer deaths are 300
• Diagnoses of cancer are 1000
• Total / population is 12 / 1000 in said society per year

Cases due to heart events

• 4 cases per 1000 face events

Rate

• Death out of cardiovascular reasons is 33%, shows most risk

Summary

• There are tests for cancer in 30% out of the group.

Population Demographic Data

Use

• Describe population
• Trends over time
• Differences in space
• Socio-economic location

Measure

• Population amount- yearly at all residents
• birth rates

Change overtime

• Based on event
• Migration
• Birth

Population is calculated

• Start and end population for a year divide by 2

birth rates for a region

• Deaths out of births * 100.

Example

• 2018 was a rate of 18.

Why is birth rate measured?

• Enable medical help for children

Rate for death and average

• Deaths out of an area divided by all people * 1000.

Rate for gaining in a population

• Gain equal growth of births and deaths

Example of gains

• 1.2 rate for Morocco
• Every 100 are 1 added with gains.