In general, three categories of measurement devices exist: passive, active, and Working Level devices. Measurement professionals have many choices of devices within each category from which to choose. The type of device used is often determined by the protocol, such as whether the test is conducted during a real estate transaction or in a large commercial building. Many factors are involved in the selection of the test device, such as cost, the ability to perform on-site analysis versus lab analysis, and ability to limit or detect tampering. Regardless of the options, measurement professionals should only use EPA or state-approved devices. A list of approved devices can be found through your state agency, the American National Standards Institute, the National Radon Proficiency Program, or the National Radon Safety Board.