Epidemiology: Basic concepts and principles ENV

Syllabus covered Introduction –Definition –Some basic concepts and terminologies Proportion, rate and ratio Prevalence Incidence Population at risk Role of the epidemiologist Sources of Health data

What is epidemiology? Epidemiology is the branch of science that deals with the study of the dynamics of health and ill health in populations

How do you define epidemiology? It is the study of the distribution and determinants of disease frequency in human population (Hennekens & Buring: Epidemiology In Medicine)

Another definition of Epidemiology __________________________ “The study of the DISTRIBUTION and DETERMINANTS of health related states and events in populations and the application of this study to the control of health problems” Last J M: A Dictionary of Epidemiology.

Underlying concepts The health/ill health continuum in a population Population at risk Population dynamics Multiple causality and multiple determinants

Population dynamics Factors influencing the population dynamic –Population pyramid –Birth & death rates –In-migration & out-migration

Population at risk A key feature of epidemiology is the measurement of disease outcomes in relation to a population at risk. The population at risk is a defined population that shares similar experience or characteristics exposed to a risk of developing disease (outcome)

About epidemiology Frequency count of disease (how many) Distribution of disease (who, where, when) The determinants of disease (what causes it, how did it happen, why did it happen)

Schematic diagram of Epidemiology Disease frequency Disease distribution The determinants of disease Descriptive epidemiology Analytic epidemiology Measures of disease frequency Measures of association Test hypothesis

Basic math in epidemiology Ratio: a/b Proportion: a/a+b Rate: (a/a+b) x t

Measures of Disease Occurences Counting cases (summarizing cases as ratio) Prevalence (proportion) Incidence (rate)

Epidemiologic measures Prevalence Incidence Interpretation of prevalence and incidence Its relationship with epidemiologic studies

Prevalence Proportion: a/a+b When expressed as a percentage: (a/a+b) x 100 i.e per 100 person Prevalence of hepatitis is 3% means 3 cases of hepatitis per 100 person For rare diseases multiply by 1,000 or 10,000

Ratios Ratio = A/B = A:B Example: In a classroom of 15 boys and 5 girls the male to female ratio is 15:5 or 3:1 In RATIO: numerator not part of denominator Odds ratio

Proportion Proportion is part of a whole Proportion = A/A+B Example: In a classroom of 15 boys and 5 girls, the proportion of female students is 5/5+15 = 25% In PROPORTION: numerator is part of the denominator Expressed per 100 = percentage Risk Ratio

Rate Rate = A/(A+B)t RATE: A proportion in which change over time is taken into account Comparison of Absolute Risk Relative Risk

Pseudo rate When the true denominator or population at risk cannot be determined. A related denominator is used as an indirect measure reflecting the population at risk Examples: Maternal mortality rate (MMR) Birth defect rate (BDR)

Pseudo rate BDR (Birth Defect Rate) Number of babies born with birth defect /Total number of deliveries (or livebirths)

Prevalence The number of existing cases (new and old) of a disease over the total population at a given point in time. Prevalence is a proportion measure and not a rate - autopsy rate - birth defect rate

Prevalence measure Surveys Cross sectional study Seroprevalence study Screening

Uses of prevalence measures __________________________ Planning for health care needs Assess public health impact Comparison of prevalence between exposed and non-exposed group is used for hypothesis generation (prevalence OR)

Incidence The number of new cases during a given time period over the total population at risk 2 types of incidence measures –Cumulative incidence –Incidence density Examples of incidence measures –mortality and morbidity rates –cause-specific mortality rates –case-fatality rate –attack rate

Incidence Number of new cases developing over a given period of time CI = P opulation at risk Number of new cases developing over a given period of time ID = Total person-time of observation

Prevalence & Incidence A ________________________ B ______________________x C __________________________________________________________ D _______________________x E ______________________x Period Point Prevalence prevalence Jan76 July76 Jan77 July77 Jan78 July78 TTR

Interrelationship between incidence and prevalence ________________________ _ P = I x D P is the prevalence I is the incidence and _ D = average duration of disease

Incidence Density (ID) The denominator differs from the CI because it take into account : Cases lost to follow-up Cases died Cases who enter locality registry later

Incidence - Quantifies number of new cases of disease that develop in a population at risk during a specified time period Three key concepts: New disease events, or for diseases that can occur more than once, usually first occurrence of disease Population at risk (candidate population) Time must pass for a person to move from health to disease

Cumulative incidence estimates the probability or risk that a person will develop disease DURING A SPECIFIED TIME. Note that the candidate population is comprised of people who are “at risk” of getting the disease Used mainly for fixed populations because its assumes that everyone is followed for the entire time period Two Types of Incidence Measures (Continued)

Example: Cumulative incidence of Sudden Infant Death Syndrome during first year of life Population1,000 livebirths Cases of SIDS 10 Cumulative Incidence10/1,000 or 1% over one year Note that all livebirths are ‘at risk’

Cumulative incidence calculation assumes that you have followed the entire population for the entire follow-up period. For example, it assumes you have followed all of the livebirths for one year or until SIDS occurred. Often you can't follow everyone for entire time period In a dynamic population, individuals enter population over time, become lost, etc. So length of follow-up is not uniform for all Incidence density do not make assumption of complete follow-up. It take into account the actual follow-up period

Incidence density (ID) = # new cases of disease person-time of observation in candidate population ID have more precision compared to CI because it directly integrates time into the denominator. However interpretation of the ID is not simple and may appear abstract.

Some Ways to Accrue 100PY 100 people followed 1 year each = 100 py 10 people followed 10 years each= 100 py 50 people followed 1 year plus 25 people followed 2 years = 100 py Time unit for person-time = year, month or day Person-time = person-year, person-month, person-day

Attack rate: # cases of disease that develop during ________defined period_______ # in pop. at risk at start of period (usually used for infectious disease outbreaks)

Further e.g of incidence rates Infectivity rate = # infected / # susceptible x 100 Pathogenicity rate = # clinical dis / # of infected x 100

Summary RateTypeNumeratorDenominator Morbidity rate IncidenceNew cases of nonfatal disease Total population at risk Mortality rate IncidenceNumber of death Total population

Summary RateTypeNumeratorDenominator Case fatality rate IncidenceNumber of deaths from a disease Number of cases of that disease Attack rateIncidenceNumber of cases of a disease Total population at risk

Summary RateTypeNumeratorDenominator Birth defect rate PrevalenceNumber of babies with a given birth defect Number of live births Disease rate at autopsy PrevalenceNumber of cases of a disease at autopsy Number of persons autopsied

Issues in the calculation of incidence _________________________ The denominator should only include persons at risk of developing disease Exclude those who already have the disease or cannot develop the disease (age, immunized against, prior removal of an organ)

Measures of disease frequency Number of persons with disease Prevalence = at a point in time Number of person in population Number of new cases of disease in a given time period Cumulative = Incidence Total population at risk Number of new cases of disease in a given time period Incidence = Density Total person-time of observation

Food-specific Attack Rate (n=60) Eat Did not eat Food served ill not ill A.R ill not ill A.R Chicken % % Anchovies % % Meatballs % % Vegetables % % Potatoes % % Cake % %