There are several different methods to collect data for statistical studies. Often, which method is best depends on the purpose of the study. A survey is when one or more characteristics of a population is studied by collecting data from every member of the sample. The data can be obtained by asking questions or making measurements. An experiment involves a group of participants undergoing a specific treatment with the goal of collecting data about their reactions to the treatment. In some statistical studies it is necessary to conduct an experiment in order to collect the data. For example, this method is often used when studying the effects of a medical treatment. An observational study involves observing the chosen sample to gather data. The researcher does not interact with the sample or expose them to treatments, like in an experiment.