How the noncentral t distribution got its hump.


Book: 
Proceedings of the Seventh International Conference On Teaching Statistics (ICOTS-7), Salvador, Brazil.
Authors: 
Cumming, G.
Editors: 
Rossman, A., & Chance, B.
Category: 
Year: 
2006
Publisher: 
Voorburg, The Netherlands: International Statistical Institute.
URL: 
http://www.stat.auckland.ac.nz/~iase/publications/17/C106.pdf
Abstract: 

Once upon a time there was only one t distribution, the familiar central t, and it was monopolised by the Null Hypotheses (the Nulls), the high priests in Significance Land. The Alternative Hypotheses (the Alts) felt unjustly neglected, so they developed the noncentral t distribution to break the monopoly, and provide useful functions for researchers-calculation of statistical power, and confidence intervals on the standardised effect size Cohen's d. I present pictures from interactive software to explain how the noncentral t distribution arises in simple sampling, and how and why it differs from familiar, central t. Noncentral t deserves to be more widely appreciated, and such pictures and software should help make it more accessible to teachers and students.