Epidemiologic basis of public health Epidemiology MPH 531 Types Study Designs in Epidemiology (DESCRIPTIVE STUDIES) Mpundu MKC MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN Detels - PH150 - 30 Sept 2009

Learning Objectives Define descriptive Epidemiology Describe the variables in Descriptive Epidemiology; Time, Place and person Understand the importance of three variables in Descriptive Epidemiology To understand why epidemiological studies are conducted To understand the concepts of different epidemiological study designs

Types of Studies Epidemiology 1. Descriptive Epidemiology Study of occurrence and distribution of disease 2. Analytic Epidemiology Studies to determine the validity of hypothesis concerning the occurrence of disease 3. Experimental Epidemiology Deliberate manipulation of the cause is predictably followed by an alteration in effect

Overview of Epidemiologic Studies 1. Descriptive Studies Population Individual Case report Case series Cross sectional studies 2. Analytic Studies Observational Case control Cohort Retrospective Prospective

Overview of Descriptive Studies 3. Interventional/Experimental Studies Randomised Controlled Trial Field trial Clinical trial

Descriptive Epidemiology Definition Descriptive epidemiology the basic type of epidemiology, which is concerned with describing the frequency and distribution of diseases and other health related conditions by time, place and person.

Descriptive Epidemiology In descriptive epidemiology, data is organised and summarised according to time, place and person. These characteristics are also referred to as epidemiologic variables Reasons for this organisation Investigator becomes familiar with the data and with the extent of the public health problem

Descriptive Epidemiology Descriptive epidemiology is the first step in epidemiological study Careful observation of available information have led to successful preventive measures. Describing health or disease occurrence according to place, person and time can lead to meaningful hypothesis formulations

Descriptive Epidemiology It provides a detailed description of the health of population Analysis of data identifies the population that are at greatest risk of acquiring a particular disease Information provides important clues to the cause of the disease and the clues can be turned into testable hypotheses

Descriptive vs. Analytic Epidemiology Descriptive Analytic Used when little is Used when insight known about a about various aspects disease of disease is available Rely on preexisting Rely on development of data new data Who, where, when Why, How Illustrates potential Evaluates the causality associations of associations

Descriptive Epidemiology What (Case definition) Who? (Persons) Where) (Place) When? (Time) How many (Measures)

Descriptive Epidemiology Study of the occurrence and distribution of disease Terms Time : When is the disease occurring? Place: Where is the diseases occurring? Person: Who is getting the disease?

Variables of Descriptive Epidemiology PERSON PLACE TIME Think of this as the standard dimensions used to track the occurrence of a disease.

Descriptive Epidemiology TIME (When) Secular Periodic Seasonal Epidemic

Time Con’t Study of disease occurrence by time is a basic aspect of epidemiologic analysis. Occurrence is usually expressed on a monthly or annual basis, daily in epidemic Some diseases occur periodically or cycles E.g. Malaria is one of the of diseases with seasonal periodicity, high peaks occur in relation to the rainy season when stagnant water are convenient for the breeding of mosquitoes

Time Disease rates change over time Some changes occur regularly and can be predicted Examining events that precede a disease rate increase or decrease, the causes can be identified and appropriate actions to control or prevent further occurrence of disease can be taken

Time Con’t Secular Trend Long term trend of disease occurrence Occurs slowly over long periods of time. Steady trend towards increase or decrease over a long period of time. Commonly use these trends to predict the future incidence of disease Used to evaluate programmes or policy decisions or to suggest what caused an increase or decrease in disease occurrence, particularly if it indicates when related event took place

Secular Trend

Secular Trend 1990 2015

Epidemiologic basis of public health Time Con’t Seasonal Variations A cyclic variation in disease frequency by time of year & season. Seasonal fluctuations Environmental factors Occupational activities Recreational activities Seasonal variations can be used to suggest possible etiology Seasonal variations can be seen for some diseases or conditions falling within calendar year Detels - PH150 - 30 Sept 2009

Seasonal Variation Seasonal variation can be seen for some diseases or conditions falling within a calendar year

Seasonal Variation . Migratory Birds? Seasonal variation can be used to suggest possible etiology

Epidemiologic basis of public health Time Con’t Point Epidemics An increase in incidence above the expected in a defined geographic area within a defined time period Short-term changes occur over limited time frame. Hours, days, wks, months To show the time course of a disease outbreak or epidemic, we use a specialized graph called an epidemic curve Used for short term exposures or diseases with short incubation period and /or illness duration Detels - PH150 - 30 Sept 2009

Point Epidemics

Epidemic Curve Number of Cases of Disease Time

Cyclic Trends Cyclic trends may be either long-term or short term events. Some are “seasonal” while others are cyclic due to other factors: Immigration School year

Cyclic Trends

Endemic, Epidemic and Pandemic Endemic: The habitual presence (or usual occurrence) of a disease within a given geographic area Epidemic: The occurrence of an infectious disease clearly in excess of normal expectance and generated from a common or propagated source Pandemic: A worldwide epidemic affecting an exceptionally high proportion of the global population

Time clustering Time Place Cluster/disease cluster Epidemiologic basis of public health Time clustering Time Place Cluster/disease cluster A group of cases occur close together & have a well aligned distribution pattern in terms of time and place Time clustering data can sometimes be used to trace the beginning to the introduction of specific casual agent Cluster analysis-used for rare or special disease events. Thalidomide and birth defects: was first marketed in Europe in the 1950s as a sleeping pill and to treat morning sickness in pregnant women Detels - PH150 - 30 Sept 2009

Place (Where) An area defined by natural boundaries may have a high or low incidence of certain diseases because it is characterized by some particular environmental or climatic conditions, e.g temperature, humidity, rainfall, altitude, mineral content of soil, or water supply. Place can assist to infer factors that increase the risk factors are present in persons (host factors) living there or in the environment or both

Place (Where?

Place (Where) Health event may be described by place to gain insight into geographical extent of the problem. e.g. place of Residence, Birth Country, Continent, district, urban, rural Frequency of disease is different in different places. These differences can occur because of the natural boundaries (e.g. forests, rivers, deserts).

Five Criteria of Place  Rate observed in all ethnic groups in the area  Rate NOT observed in persons of similar groups inhabiting other areas Healthy persons entering area get ill at same frequency People who leave do NOT show similar levels Similar levels of infestation in other species (if zoonotic disease)

Place Place gives an idea of where the agent that causes disease normally lives, multiplies, what may carry it and how it spreads Where are the rates higher? Lower? Geographic location of source Geographic location of reservior

Epidemiologic basis of public health Spot maps show location of individual cases Detels - PH150 - 30 Sept 2009

Characteristics Relating to Place International Variation within countries Urban-rural Local

Person WHO is getting the disease? Many variables are involved and studied, but factors such as sex, age & race often have a major effect.

Person Age Hobbies Gender Hereditary Marital status Ethnicity/Race Occupation Travel Immunization status Personal Habits Underlying disease Stress Medication Family unit Nutritional status School Socioeconomic factors Genetics Crowding Lifestyle Behaviour Religion/Culture

Age (Who?)

Sex (Who?)

Age (Who?)

Summary of Descriptive epidemiology : Patterns of Disease Occurrence Distribution of disease in populations By “person” : age, race / ethnicity, gender, occupation, education, marital status, genetic marker, sexual preference By “place” : residence (urban vs. rural), worksite, social event By “time” : week, month, year: sporadic, seasonal, trends

Procedure in conducting a Descriptive Study Defining the population Defining the disease under study Describing the disease by Time Place Persons Measurements of the disease Comparing with known indices Formulation of an aetiological hypothesis