Weidacker, K., Ehring, T., Zetsche, U., Wahl, K., Schönfeld, S., & Ehlers, A. (2011). The Perseverative Thinking Questionnaire (PTQ): Validation of a content-independent measure of repetitive negative thinking. Journal of Behavior Therapy and Experimental Psychiatry, 42(2), pp. 225-232. doi:10.1016/j.jbtep.2010.12.003

Weidacker, Kathrin, Thomas Ehring, Ulrike Zetsche, Karina Wahl, Sabine Schönfeld, and Anke Ehlers. "The Perseverative Thinking Questionnaire (PTQ): Validation of a Content-independent Measure of Repetitive Negative Thinking." Journal of Behavior Therapy and Experimental Psychiatry 42, no. 2 (2011): 225-232.

Weidacker, Kathrin, et al. "The Perseverative Thinking Questionnaire (PTQ): Validation of a Content-independent Measure of Repetitive Negative Thinking." Journal of Behavior Therapy and Experimental Psychiatry 42.2 (2011): 225-232.