In 1914, a federal doctor walked an asylum in Mississippi where the patients caught a deadly disease and the nurses never did. Part four of four: my four questions about what a house, a clinic and a bill can do for a body, each answered by finding its nurses, the comparison group its method needs.
In 1854, two water companies ran their pipes down the same London streets, and the houses on one company's water died of cholera at eight times the rate of their neighbors. Part three of four: my three questions about the body each get a method that answers them directly, and an answer counts only if it arrives in time to change something.
Thirty-nine people who never set foot in a Philadelphia hotel in 1976 caught the illness spreading through a veterans' convention inside, and pointed investigators to the air. Part two of four: before any data, I decide who is compared and what is measured for each of my four questions about the healthspan gap, the way the investigators did on the sidewalk.
In July 1995, Chicago's medical examiner ran out of room for the dead, and three experts named three different killers. Part one of four: the biostatistics question I keep coming back to has four true answers at once, and it still splits cleanly into what a body can do and what its surroundings ask of it.