1. Becker, P. (1985). Sinnfindung als zentrale komponente seelischer gesundheit. In A. Längle (Ed.), Wege zum sinn. Logotherapie als orientierungshilfe (pp. 186–207). Munich: Piper.
2. Crumbaugh, J. C. (1968). Cross-validation of purpose-in-life-test based on Frankl’s concepts. Journal of Individual Psychology, 24, 74–81.
3. Crumbaugh, J., & Carr, G. (1979). Treatment of alcoholics with logotherapy. International Journal of the Addictions, 14, 847–853.
4. Crumbaugh, J. C., & Henrion, R. (1988). The PIL test: Administration, interpretation, uses, theory and critique. The International Forum for Logotherapy, 11, 76–88.
5. Crumbaugh, J. C., & Maholick, L. T. (1964). An experimental study in existentialism: The psychometric approach to Frankl’s concept of “noögenic neurosis”. Journal of Clinical Psychology, 20, 200–207. https://doi.org/10.1002/1097-4679(196404)20:2200:AID-JCLP2270200203[3.0.CO;2-U].