1. Mahl GF (1959) Exploring emotional states by content analysis. In: Pool I (ed) Trends in content analysis. University of Illinois Press, Urbana
2. Holsti O (1969) Content analysis for the social science and humanities. Addison-Wesley, Reading
3. Krippendorf K (1980) Content analysis. Sage, Beverly Hills
4. Popper KR (1959) The logic of scientific discovery. Basic Books, New York
5. Berelson B (1952) Content analysis in communication research. Glencoe, New York, p 31