Epidemiology HUNG-EN SUNG and CASSANDRA RAMDATH City University of New York, USA Epidemiology is the study of the distri- bution of disease as well as the study of its determinants and its consequences for human populations (Rothman 2012). It uses empirical methods and conceptual models to answer questions such as how much disease there is, what speciic factors put individuals at risk, and how severe the outcomes of dis- ease are for patient populations, in order to inform public health policymaking. he term “disease” encompasses not only physical or mental illnesses but also behavioral patterns with negative health consequences, such as substance abuse or violence. he measurement of disease occurrence begins with the estimation of incidence and prevalence. Disease incidence is the num- ber of new cases in a population within a speciic period of time. First-ever incidence picks up only irst-ever onsets; by contrast, episode incidence records all onsets of disease events, including those of recurrent episodes. Cumulative incidence expresses the risk of contracting a disease as the proportion of the population that would experience the onset over a speciic time period. Prevalence is the number of people in a population with a speciic disease. Point prevalence counts all diseased individuals at a point in time, whereas period prevalence records those with the disease during a stated time period. Cumulative prevalence includes all those who contracted the disease dur- ing their lives or between two speciic time points. he nature of the disease itself deter- mines the appropriate choice of measure. For he Wiley Blackwell Encyclopedia of Race, Ethnicity, and Nationalism, First Edition. Edited by John Stone, Rutledge M. Dennis, Polly S. Rizova, Anthony D. Smith, and Xiaoshuo Hou. © 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd. DOI: 10.1002/9781118663202.wberen666 example, for single-episode conditions with a clearly deined onset such as chickenpox, irst-ever and cumulative incidence rates are most useful, but, for recurrent conditions with ill-deined onsets such as allergy, period and cumulative prevalence rates are most oten analyzed. Population epidemiology attempts to unravel causal mechanisms of disease with a view to prevention. Since most diseases are determined by multiple genetic and environmental factors, exposure to single risk factors is usually neither a suicient nor a necessary cause of disease. Consequently, eforts are devoted to quantifying the level of increased risk when one is exposed to a particular risk factor. Risk is normally mea- sured as either the ratio of the prevalence of disease in two populations or the ratio of the odds of exposure to a particular risk factor between two groups. Clinical epidemiology, in turn, aims at identifying disease outcomes with the goal to control the damage done to patients. Disparities in health and health care across racial, ethnic, and socioeconomic backgrounds in the United States and other countries are well documented (Barr 2008; Lenard and Straehle 2012). Social disadvan- tages and failures routinely translate into higher rates of morbidity, disability, and mor- tality as well as into lower rates of medical service utilization. Unfortunately, the lack of individually speciic data undermines the establishment of reliable baseline and trend analyses of health, health-care access, cost, and quality by group characteristics. A better understanding of the causes of these diferences requires the availability of quality data on race, ethnicity, socio- economic status (SES), acculturation and language, and sociopolitical capital—for