In the previous section, we noted that several different methods are usually available for measuring any particular variables. How can we decide which method is best? Whenever the variable is a hypothetical construct, a researcher must use an operation as a measurement procedure. Inessence, an operation definition is an indirect method of measuring something that cannot be measured directly. How can we be sure that the measurements obtained from an operational definition actually represent the intangible construct? In general, we are asking how good a measurement procedure, or measure, is. Researchers have developed two general criteria for evaluation the quality of any measurement procedure : validity and reliability