The dielectric loss factor tan (delta) is a critical parameter in transducer design and performance prediction, as it is directly related to the electrical energy lost to Joule heating. A method for calculating the equivalent loss factor of 'quasi linear' materials from a measured major polarization vs. field loop by extending the standard definition of tan (delta) for a linear lossy capacitor to nonlinear materials is presented. To extract effective loss tangents for minor loops from the major loops, an area correction algorithm was implemented. This algorithm proves to be nearly exact for all bias and drive levels in the case of an ideal linear capacitor. In most cases, the effective tan (delta) calculated from a major loop agrees fairly well with that calculated from a directly measured minor loop. Finally, the behavior of the loss tangent as a function of the dc bias field, ac drive field, prestress level and temperature will be examined. It shall be shown that, in general, the effective loss factor of lead magnesium niobate-lead titanate decreases with increasing temperature, consistent with its transition from a piezoelectric to an electrostrictive material, but increases with prestress. At a fixed temperature and prestress level, the loss factor increases as the bias or drive levels decrease. However, at certain bias levels, the loss tangent is practically the same, regardless of the drive level.