Letter to the editor

References 

1. Cook TD, Campbell DT. Quasi-experimentation: Design and analysis issues for field settings. Boston: Houghton Mifflin; 1979;
2. Mook DG. In defense of external invalidity. Am Psychol. 1983;32:379–387
3. Peirce, C. S. (1899). The scientific attitude and fallibilism. In C. Hartshorne, & P. Weiss (Eds.) (1931-35). Collected papers of Charles Sanders Peirce (CP I, pp. 135–49). Cambridge, MA: Harvard University Press.