The main reasons for this is to avoid trials in the later phases using doses that are significantly different from those that will subsequently be recommended for clinical use and also to avoid the need for modification of dosing schedules at later stages where a large amount of data has already been accumulated for a different dose range.[2] The duration of action should be determined during dose-ranging study, as it will allow definition of the dosage schedule. Because it is hard to measure reliable pharmacodynamic parameter, it is difficult to determine the duration of action during early clinical trials. Other parameters instead are suggested as a tentative dosage, such as half-lives in plasma and urine in various test species and human, receptor binding in vitro, or pharmacodynamic data in vivo in animals.[3]