A researcher is interested in whether drinking water right before bed increases the likelihood that children will wet their bed at night. The researcher recruits fifty 3-year-old children to participate in her study. Twenty-five children are given two glasses of water two hours before their bedtime (one glass per hour) and twenty-five children are prohibited from consuming liquids within two hours of their bedtime. Then, the researcher called the parents asking them to report whether their child wet the bed at night. What method of data collection was employed in this study?