Introduction To Epidemiology Mr. Manuel

What is Epidemiology?

Definition of Epidemiology Last JM: A Dictionary of Epidemiology The study of the distribution and determinants of health related states and events in populations and the application of this study to control of health problems

What Is The Unique Skill Of Epidemiologists? Measuring disease frequency in populations

Measuring Disease Frequency Has Several Components Classifying and categorizing disease Deciding what constitutes a case of disease in a study Finding a source for ascertaining the cases Defining the population at risk of disease Defining the period of time of risk of disease Obtaining permission to study people Making measurements of disease frequency Relating cases to population and time at risk

Two Broad Types of Epidemiology DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of time, place, and person. Typical study design: community health survey (approximate synonyms - cross-sectional study, descriptive study) Testing a specific hypothesis about the relationship of a disease to a putative cause, by conducting an epidemiologic study that relates the exposure of interest to the disease of interest. Typical study designs: cohort, case-control

The Basic Triad Of Descriptive Epidemiology THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY ARE: PERSON PLACE TIME

Person (characteristics?) Age Gender Socio-economic status (education, occupation, income) Marital status Ethnicity/race/genetic profile Behavior / habits 8

Place (where ?) Geographically restricted or widespread (outbreak, epidemic, pandemic)? Off-shore (tsunami…) Climate effects (temperature, humidity, combined effects..) Urban / sub-urban-squatter / rural   Relation to environmental exposure (water, food supply, etc)   Multiple clusters or one? 9

Time (when ?) Changing or stable? Clustered (epidemic) or evenly distributed (endemic)? Time-trends: Point source, propagated, seasonal, secular, combinations 10

Data Collection Methods Primary: where the investigator is the first to collect the data. Sources include: medical examinations, interviews, observations, etc. Pros: less measurement error, suits objectives of the study better. Cons: costly, may not be feasible. Secondary: where the data is collected by OTHERS, for other purposes that those of the current study. Sources include: individual records (medical / employment); group records (census data, vital statistics)

Descriptive Epidemiology Is A Necessary Antecedent Of Analytic Epidemiology To undertake an analytic epidemiologic study you must first: Know where to look Know what to control for Be able to formulate hypotheses compatible with laboratory evidence

A common error in epidemiology is moving to analytic epidemiology without having a solid base in the descriptive epidemiology of the condition.

public health, because of the emphasis on disease prevention. Epidemiologists are required to have some knowledge of the disciplines of the following: public health, because of the emphasis on disease prevention. clinical medicine, because of the emphasis on disease classification and diagnosis. 

pathophysiology, because of the need to understand basic biological mechanisms in disease. statistics, because of the need to quantify disease frequency and its relationships to antecedents. social sciences, because of the need to understand the social context in which disease occurs and presents.

Purposes Of Epidemiology Identify causes and risk factors for disease. Determine the extent of disease in the community. Study natural history and prognosis of disease. Evaluate preventive and therapeutic measures Provide foundation for public policy

EVERY HEALTH OUTCOME HAS SOME INTERESTING AND USEFUL EPIDEMIOLOGIC CHARACTERISTIC DEATH RATES BY SOCIAL CLASS FROM A CERTAIN CAUSE AMONG 1,316 PEOPLE Men Women Children Total 1st class 67% 3% 38% 2nd class 92% 14% 59% 3rd class 84% 54% 66% 62% 82% 26% 48% WHAT CAUSE OF DEATH IS THIS?

The previous slide shows death rates by class of ticket on the Titanic, a large ocean liner that sank after colliding with an iceberg in 1912

Pandemic Influenza Phases Time: Mondays and Wednesdays 4:10-5:20 p.m. Office hours: BY ARRANGEMENT Place: Room A -131 East Fee Hall Department of Epidemiology classroom. Person: Nigel Paneth, Instructor. 353-8623; paneth@msu.edu

WHO phases of pandemic alert - Phase 4 Verified human-to-human transmission of an animal or human-animal influenza able to cause “community-level outbreaks.” The ability to cause sustained disease outbreaks in a community. Indicates a significant increase but not necessarily pandemic

Phase 5 Human-to-human spread of the virus into at least two countries in one WHO region. While most countries will not be affected at this stage Is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication, and implementation of the planned mitigation measures is short.

Phase 6 (pandemic phase) Community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Indicate that a global pandemic is under way.

Differences Between Laboratory Sciences And Field Sciences In the Field: Mostly observational  Variables controlled by nature  Some variables unknown  Replication difficult; exact replication impossible  Results often uncertain  Meaning of results for humans clear  Statistical control often very important  Highly labor intensive

Differences Between Laboratory Sciences And Field Sciences In the Laboratory: Mostly experimental Variables controlled by the investigator All variables known Replication easy Results valid Meaning of results for humans uncertain. Little need for statistical manipulation of data. Highly equipment intensive

The Basic Triad Of Analytic Epidemiology THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE: HOST Define host, agent? AGENT ENVIRONMENT

Host Age Socio-economic status Gender Ethnicity/Race Behavior

Host Factors Genetic endowment Immunologic state Age Personal behavior

Agent Living/Non-living Species Behavior Habitat If the agent is human, gender, race etc. are factors Examples: Nutrients, Poisons, Allergens, Radiation, Physical trauma, Microbes, Psychological experiences

Environment Crowding Atmosphere Modes of communication – phenomena in the environment that bring host and agent together, such as: Vector Vehicle Reservoir

Environment Geographically restricted or widespread (pandemic)? Relation to water or food supply.   Multiple clusters or one?

Analytic Epidemiology Responses to this question vary. As outlined below, some believe that altruistically, society should pursue disease prevention to improve health and save money. These are noble goals, but I will demonstrate later that prevention does not always save money. The desire to prevent NCDs probably is a repercussion of our efforts to prevent infectious diseases. We have had great success in preventing communicable diseases. This has led to an initiative to prevent NCDs.

Study Development Process Descriptive Studies: Data Collection and Analysis Model Building and hypothesis formulation Analyze results and retest Analytic Studies for Hypothesis testing

Study Designs - Analytic Epidemiology Experimental Studies Randomized controlled clinical trials Community trials Observational Studies Group data Ecologic Individual data Cross-sectional Cohort Case-control An Introduction to Epidemiology (CDC) http://www.cdc.gov/excite/classroom/intro_epi.htm

Experimental Studies treatment and exposures occur in a “controlled” environment planned research designs clinical trials are the most well known experimental design. Clinical trials use randomly assigned data. Community trials use nonrandom data Experimental studies are stronger in determining the etiology of disease than descriptive studies

Observational Studies non-experimental observational because there is no individual intervention treatment and exposures occur in a “non-controlled” environment individuals can be observed prospectively, retrospectively, or currently

Cross-sectional studies An “observational” design that surveys exposures and disease status at a single point in time (a cross-section of the population) Cross section al studies are some of the first studies completed because of ease and low cost time Study only exists at this point in time

Cross-sectional Studies Often used to study conditions that are relatively frequent with long duration of expression (nonfatal, chronic conditions) It measures prevalence, not incidence of disease Example: community surveys Not suitable for studying rare or highly fatal diseases or a disease with short duration of expression Cross-sectional studies involve point prevalence, not incidence. For very infrequent diseases they are of limited utility

Cross-sectional Design factor present No Disease factor absent Study population factor present Disease factor absent Cross-sectional studies examine a point in time time Study only exists at this point in time

Epidemiologic Study Designs Case-Control Studies an “observational” design comparing exposures in disease cases vs. healthy controls from same population exposure data collected retrospectively most feasible design where disease outcomes are rare Case-control studies in epidemiology are the most used type of study design

Case-Control Studies Cases: Disease Controls: No disease Case-Control studies represent one form of analytic study that provides information on the relationship between causal factors and injuries. In a case-control study, subjects who have been injured are identified and their past exposure to suspected causal factors is compared with that of controls (persons who have not been injured). Many case-control studies ascertain exposure from personal recall, using either a self administered questionnaire or an interview. The validity of such information will depend in part on the subject matter. People may be able to remember recent events quite well. On the other hand, long term recall is generally less reliable. Source: Chapter 8: Case-control and cross-sectional studies, Epidemiology for the Uninitiated

Case-Control Studies Strengths Less expensive and time consuming Efficient for studying rare diseases Limitations Inappropriate when disease outcome for exposure is not known at start of study Exposure measurements taken after disease occurrence Disease status can influence selection of subjects Case control studies provide low cost answers to health questions.

Case-Control Design Study begins here factor present Cases (disease) factor absent Study population factor present Controls (no disease) factor absent Case-Control Design present past time Study begins here

Epidemiologic Study Designs Cohort Studies an “observational” design comparing individuals with a known risk factor or exposure with others without the risk factor or exposure looking for a difference in the risk (incidence) of a disease over time best observational design data usually collected prospectively (some retrospective) The cohort studies is the best for observational studies as the environmental event can be assessed before any disease outcome

Cohort Design Study begins here disease Factor present no disease population free of disease disease Factor absent no disease Cohort Design present future A cohort studies follows a cohort of individuals who do not have disease, and then identified over time those individuals who have an outcome time Study begins here

Timeframe of Studies Prospective Study - looks forward, looks to the future, examines future events, follows a condition, concern or disease into the future Prospective cohort study From Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Prospective_cohort_study time Study begins here

Prospective Cohort study Exposed Outcome Measure exposure and confounder variables Non-exposed Outcome Baseline Case-control studies are perhaps the most frequent form of analytic study design. These designs are very good for events that are rare in occurrence.. Still, there are some situations where cohort study designs would be appropriate in the field. The classic design in a cohort study is shown here. The study begins by assessing baseline levels of the exposure and other variables. Study subjects are then followed on a regular basis to identify the outcome. The frequency of outcomes are tested between persons who had exposure to the possible risk factor at baseline and persons with no exposure. time Study begins here

Timeframe of Studies Retrospective Study - “to look back”, looks back in time to study events that have already occurred time Study begins here Prospective vs. retrospective studies http://www.statsdirect.com/help/basics/prospective.htm

Retrospective Cohort study Exposed Outcome Measure exposure and confounder variables Non-exposed Outcome Baseline An alternative form of the cohort study is something termed the retrospective cohort study. Other researchers may also call this a historical prospective study. This design is nearly identical to the prospective cohort study. The sequence of baseline exposure determination and longitudinal follow-up for outcomes is similar. The difference lies in the time in which the study begins. In this retrospective design, the researcher constructs the cohort study by looking back in time and placing data in the appropriate order and sequence. These studies are possible to do with large medical databases, such as the membership files of the Health Maintenance Organizations, or the medical files in the Scandinavian countries. time Study begins here

Cohort Study Strengths Exposure status determined before disease detection Subjects selected before disease detection Can study several outcomes for each exposure Limitations Expensive and time-consuming Inefficient for rare diseases or diseases with long latency Loss to follow-up -- Exp. Measured before disease - so no temporal ambiguity -- Exposure measured before disease - so disease cannot influence the amount of error with which exposure status is measured -- Subject selection before disease, disease status does not influence of subjects

Experimental Studies investigator can “control” the exposure akin to laboratory experiments except living populations are the subjects generally involves random assignment to groups clinical trials are the most well known experimental design the ultimate step in testing causal hypotheses Experimental studies are the ultimate form of design in assessing causality as there is random assignment to groups.

Experimental Studies In an experiment, we are interested in the consequences of some treatment on some outcome. The subjects in the study who actually receive the treatment of interest are called the treatment group. The subjects in the study who receive no treatment or a different treatment are called the comparison group. Experimental Studies http://www.mcs.anl.gov/~itf/dbpp/text/node31.html

Epidemiologic Study Designs Randomized Controlled Trials (RCTs) a design with subjects randomly assigned to “treatment” and “comparison” groups provides most convincing evidence of relationship between exposure and effect not possible to use RCTs to test effects of exposures that are expected to be harmful, for ethical reasons RCT should be conducted for hypotheses that have not been tested. Great care must be taken to evaluate possible negative outcomes as well as positive outcomes

Epidemiologic Study Designs Randomized Controlled Trials (RCTs) the “gold standard” of research designs provides most convincing evidence of relationship between exposure and effect trials of hormone replacement therapy in menopausal women found no protection for heart disease, contradicting findings of prior observational studies It is not unexpected to find that observational studies find different results than for clinical trials. For example there have been 100s of observational studies demonstrating that hormone replacement was protective for women. However, when this was put to a clinical trail, the surprising result was that hormone replacement was not protective

Experimental Design time Study begins here (baseline point) outcome RANDOMIZATION Intervention no outcome Study population outcome Control no outcome Experimental Design baseline future Experimental and observational studies A common goal for a statistical research project is to investigate causality, and in particular to draw a conclusion on the effect of changes in the values of predictors or independent variables on dependent variables or response. There are two major types of causal statistical studies: experimental studies and observational studies. In both types of studies, the effect of differences of an independent variable (or variables) on the behavior of the dependent variable are observed. The difference between the two types lies in how the study is actually conducted. Each can be very effective. An experimental study involves taking measurements of the system under study, manipulating the system, and then taking additional measurements using the same procedure to determine if the manipulation has modified the values of the measurements. In contrast, an observational study does not involve experimental manipulation. Instead, data are gathered and correlations between predictors and response are investigated. From Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Statistics time Study begins here (baseline point)

Randomized Controlled Trials Disadvantages Very expensive Not appropriate to answer certain types of questions it may be unethical, for example, to assign persons to certain treatment or comparison groups Understanding controlled trials: Why are randomised controlled trials important? By Bonnie Sibbald and Martin Roland http://www.bmj.com/content/316/7126/201.full

Credits All information modified from these sources: Nigel Paneth at the University of Pittsburgh Thomas Songer, PhD - University of Pittsburgh Modified by Supercourse team Ahmed Mandil, MBChB, DrPH Prof of Epidemiology High Institute of Public Health University of Alexandria Understanding controlled trials: Why are randomised controlled trials important? By Bonnie Sibbald and Martin Roland http://www.bmj.com/content/316/7126/201.full