Statistical data are elemental building blocks of scholarly inquiry. They provide support for hypotheses and arguments. Understanding how to collect and use data correctly is critical to scientific analysis. The Concise Corsini Encyclopedia of Psychology and Behavioral Science contains this background article on Psychometrics.
More coverage of statistical topics is at: VassarStats: Website for Statistical Computation. This is a useful and user-friendly tool for performing statistical computation. Links show an annotated list of the statistical procedures available under each rubric (e.g. distributions, ordinal data).