Epidemiology Study Guide

Introduction to Epidemiology

History * John Graunt * Published Bills of Mortality in 1622 * Analysis of weekly reports of births and deaths in London by sex, age, and time. * Discovered that births and deaths of men occurred in excess; high death rates of infants; and seasonal pattern with highest mortality in winter. * John Snow * Conducted one of the first observational studies in the neighborhoods of 19th century London and discovered that contaminated drinking water was the cause of cholera. * Carefully documented what he called a “natural experiment” in which neighbors received water provided by different companies and had differing rates of disease. * Richard Doll and Austin Bradford Hill * Conducted groundbreaking studies on cigarette smoking and lung cancer in the 1950s. * James Lind * Conducted one of the earliest experimental studies, which was the treatment of scurvy among sailors. * Using sound experimental principles, he found that the consumption of oranges and lemons were the most effective remedies for scurvy in this population. * William Farr * Compiled the Statistical Abstracts in Great Britain from 1839 through 1880. * He pioneered many activities encompassed by modern epidemiology, including the calculation of mortality rates using census data for denominators.

Definitions * Epidemiology – the study of the distribution and determinants of disease frequency. It is an applied science whose primary mission is to identify targets for preventative interventions. It rests on two assumptions * Disease does NOT occur randomly * Causal and preventative factors can be identified through systematic investigation. * Essential Elements of the Epidemiologic Approach 1. Human participants 2. Systematic investigation 3. Quantitative assessment 4. “Natural experiments” 5. Particular interest in modifiable causes

Definitions cont. * What Can Epidemiology Do? * Assess the extent and importance of a disease * Describe the natural history of a disease * Describe disease outcomes * Identify the causes or etiology of a disease * Evaluate public health and medical interventions * Inform policy * Descriptive Epidemiology – focuses on describing patterns of disease occurrence * Analytic Epidemiology – assesses the associations between risk factors and disease incidence or outcomes through quantitative estimates of association and formal statistical tests of hypotheses. * Determinants of Disease – any factors that directly or indirectly affect the frequency of disease occurrence in a population because they cause disease or predispose to disease, or they protect against disease or prevent disease from occurring. They can operate at the level of the individual or the community and can be characteristic of the host, agent/exposure, or environment. * Epidemiologic Triad – Environment, Host, Agent * Epidemiologic Reasoning 1. Suspicion that a factor influences disease occurrence. Arises from clinical practice, lab research, examination of disease patterns by person, place and time, or prior epidemiologic studies. 2. Formulate specific hypothesis. 3. Conduct a study to determine the relationship between the exposure and the disease. Need to consider chance, bias, confounding when interpreting the study results. 4. Judge whether association may be causal. Need to consider findings of other studies, strength of the association, timing of exposure and agent. * Cohort Study – Subjects are chosen on the basis of exposure status and followed to assess the occurrence of disease. * Experimental Study – Special type of cohort study in which the investigator assigns the exposure to individuals, preferably at random. * Case-Control Study – Cases with the disease and controls without the disease are chose and past exposure to a factor is determined. * Random Error – leads to a false association between the exposure and the disease due to chance. * Bias – any source of error in the determination of the association between the disease and exposure * Selection bias or information bias. * Confounding – a situation in which a correlate of a causal factor, which does not itself cause disease, may wrongly be identified as a disease. * Public Health – A multidisciplinary field whose goal is to promote the health of populations through organized community efforts. * Measure of Disease Frequency – quantifies how often a disease arises in a population. * Disease Distribution – refers to the pattern of disease according to the characteristics of person, place, and time.

Measures of Disease Frequency

* Why Quantify the Occurrence of Disease? * To know what level of resources are required to respond appropriately * To estimate the probability that an individual may get a disease * To make systematic and valid comparisons across geographic regions, over time, and within population subgroups. * Population – group of people with a common characteristic like age, race, sex, or geographic location. * Fixed – cohort, membership is permanent and defined by an event * Dynamic – membership is transient, defined by being in/out of a “state” * Epidemiology cohort – a group of individuals followed over time. * Disease Frequency – used to quantify disease occurrence in a population. Measures of disease frequency should take into account: * Number of individuals affected with the disease * Size of source population * Length of time the population was followed * Three Classes of Mathematical Parameters – used to relate number of cases of disease to the size of population and time, must specify if it measures events or people. * Ratio – division of one number by another, numbers don’t have to be related. * Proportion – numerator is subset of denominator, often expressed as a percentage. * Rate – time is an intrinsic part of denominator * Prevalence – Quantifies number of existing cases of disease in a population at a point or during a period of time. * P = # of existing cases / # in total population * Point prevalence – prevalence at a particular point in time. * Period prevalence – prevalence over a period of time. * Interpretation: Probability a person selected at random from the population will have the condition. * Incidence – Quantifies number of new cases of disease that develop in a population at risk during a specified time period. * Population at risk can’t have disease already, should have relevant organs * Time must pass for a person to move from health to disease. * Incidence Proportion * IP = # new cases / # at risk * An estimate of the probability or risk that a person will develop disease during a specified time. . * Used mainly for fixed populations because it assumes that everyone is followed for the entire time period. * AKA cumulative incidence, risk, attack rate. * Limitations: assumes that you have followed the entire population for the entire follow-up period. * Incidence Rate * IR = # new cases / person-time of observation * Interpreted as the speed in which events occur. * In closed population: IR = # new cases / # at risk X duration of observation * In open population: IR = # new cases / sum of person-times. * Relationship between Prevalence and Incidence * P/(1-P) = IR x D * Prevalence depends on incidence rate and duration of disease * If incidence is low but duration is long, prevalence is high * If incidence is high but duration is short, prevalence is low * Conditions for equation to be true * Steady state, IR constant, Distribution of durations constant. * Figuring Duration from prevalence and incidence * Duration = P/IR