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La segunda edición de este libro llega cuando todavía nos afecta la pandemia de covid-19 y en los medios proliferan los epidemiólogos de salón o de Wikipedia que interpretan números mágicos que no comprenden (incidencia acumulada, R…) o que atribuyen voluntad al virus y pretenden doblegar la curva (doblegar: hacer a alguien que desista de un propósito y se preste a otro [DRAE]). Una parte importante de la epidemiología es cuantificar, pero comprendiendo qué se hace, qué significan las fórmulas y los números y cómo interpretar el resultado. En nuestra experiencia enseñando Epidemiología, los alumnos señalan continuamente la necesidad de disponer de mayor número de problemas resueltos, lo que nos decidió hace unos años a la publicación de este libro. En esta edición hemos ampliado especialmente los capítulos dedicados a los tipos de estudio, sesgo de confusión y pruebas diagnósticas. El libro está ordenado de la misma forma que nuestro temario –que resulta bastante clásico y similar al utilizado en otras Facultades–. Se comienza con problemas sobre las medidas de frecuencia de enfermedad (riesgo, tasa de incidencia y prevalencia) que sirven de base para los capítulos segundo (medidas de asociación e impacto: riesgo relativo, odds ratio, fracciones atribuibles...) y tercero (tipos de estudio epidemiológico). Este tercer capítulo ocupa –como es natural– casi un tercio del total del libro. En el cuarto capítulo se aborda uno de los problemas centrales de la Epidemiología y uno de los que más dolores de cabeza producen al alumno (y posteriormente al investigador): el sesgo de confusión. Los capítulos quinto y sexto tratan –de forma más breve– otros tipos de sesgo. La validación de pruebas diagnósticas es objeto de dos capítulos: séptimo, sobre reproducibilidad (grado de acuerdo, índice kappa, gráficos de Bland-Altman) y octavo, sobre validez (sensibilidad, especificidad, valores predictivos, razones de verosimilitud). Los dos últimos capítulos se dedican a la aplicación de los conceptos previos a las enfermedades infecciosas y la investigación de epidemias. Finalmente, se presentan soluciones detalladas de todos los problemas del libro. La terminología usada en Epidemiología no es uniforme. Por ello, cada capítulo comienza con un cuadro que resume los términos clave, sus sinónimos y los correspondientes términos en inglés.