The research of transpersonal psychology is based upon both quantitative and qualitative methods, but some commentators have suggested that the main contribution of transpersonal psychology has been to provide alternatives to the quantitative methods of mainstream psychology. Although the field has not been a significant contributor of empirical knowledge on clinical issues, it has contributed important quantitative research to areas such as the study of meditation. One of the demarcations in transpersonal theory is between authors who are associated with hierarchical/holarchical, sequential, or stage-like models of human development, such as Ken Wilber and John Battista, and authors who are associated with Jungian perspectives, or models that include the principle of regression, such as Michael Washburn and Stanislav Grof. The transpersonal psychology of Ken Wilber is often mentioned as an influential theoretical framework for the field. Wilber is often regarded as a leading theorist and pioneer of the transpersonal movement, but he has not been actively associated with the label for quite some time. Several commentators note that he has distanced himself from the transpersonal field in favour of a new model that he calls integral. However, his psychological model still remains influential to the practice and development of transpersonal psychology, and transpersonal themes remain a central part of his own work. Central to his theory of consciousness is a synthesis of eastern and western psychologies and models of human development. Wilber's model of consciousness consists of three broad developmental categories: the prepersonal or pre-egoic, the personal or egoic, and the transpersonal or trans-egoic. A more detailed version of this model includes nine different levels of human development, in which levels 1-3 are pre-personal levels, levels 4-6 are personal levels and levels 7-9 are transpersonal levels. Later versions also include a tenth level. The transpersonal stages, or the upper levels of the model, are the home of spiritual events and developments. The framework proposed by Wilber suggests that human development is a progressive movement through these stages of consciousness. The theory implies that different schools of psychology are associated with different levels of the model, and that each level of organization, or self-development, includes a vulnerability to certain pathologies associated with that particular level. Each level also represents developmental tasks that must be properly met, or they might lead to developmental arrest. A basic tenet of Wilber's transpersonal psychology is a concept called the "pre/trans fallacy". That is, a confusion of transpersonal progression with prepersonal regression. According to writers in the field western schools of psychology have had a tendency to regard transpersonal levels as pathological, equating them with regressive pathological conditions belonging to a lower level on the model.