Special Forms of Correlational ResearchEpidemiological studies and longitudinal studies are two kinds of correlational researchused widely by clinical investigators. Epidemiological studies reveal the incidenceand prevalence of a disorder in a particular population. Incidence is the number ofnew cases that emerge during a given period of time. Prevalence is the total numberof cases in the population during a given time period; prevalence includes both existingand new cases. Many researchers also refer to epidemiological studies as "descriptivestudies" because the goal of such investigations is largely to describe the incidence orprevalence of a disorder "without trying to predict or explain when or why it occurs"(Compas & Gotlib, 2002, p. 69).Over the past 30 years clinical researchers throughout the United States have workedon the largest epidemiological study ever conducted, called the Epidemiologic CatchmentArea Study. They have interviewed more than 20,000 people in five cities todetermine the prevalence of many psychological disorders and the treatment programsused (Eaton et aI., 2007; Narrow et aI., 2002; Regier et aI., 1993). Two other large-scaleepidemiological studies in the United States, the National Comorbidity Survey and theNational Comorbidity Survey Replication, have questioned more than 9,000 individuals(Druss et aI., 2007; Kessler et aI., 2007, 2005, 2003). These studies have been furthercompared with epidemiological studies of specific groups, such as Hispanic and AsianAmerican populations, or with epidemiological studies conducted in other countries,to see how rates of mental disorders and treatment programs vary from group to groupand from country to country (Alegria et aI., 2007, 2004, 2000; Kessler et aI., 2006).Such epidemiological studies have helped researchers identity groups at risk forparticular disorders. Women, it turns out, have a higher rate of anxiety disorders anddepression than men, while men have a higher rate of alcoholism than women. Elderlypeople have a higher rate of suicide than young people. Hispanic Americans experienceposttraumatic stress disorder more than other racial and ethnic groups in the UnitedStates. And persons in some countries have higher rates of certain mental disordersthan those in other countries. Eating disorders such as anorexia nervosa, for example,appear to be more common in Western countries than in non-Western ones. Thesetrends may lead researchers to suspect that something unique about certain groups orsettings is helping to cause particular disorders. Declining health in elderly people, forexample, may make them more likely to commit suicide. Similarly