A medical researcher interested in asthma symptoms wanted to know how symptoms were affected in dry versus humid conditions. The researchers recruited 18 asthma patients to spend four weeks under two conditions: sleeping with a dehumidifier for two weeks to create a "dry" environment and sleeping with a humidifier for the remaining two weeks to create a "humid" environment. Patients were asked to rate their symptoms at regular intervals using a scale from "0 - no symptoms" to "20 - maximum asthma symptoms." The change in asthma symptoms from dry to humid conditions was 5.82, showing an advantage for humid conditions. What was the sample in this study?