Selection bias is a persistent, and often hidden, problem in educational research. It is the primary obstacle standing in between increasingly available large education datasets and the ability to make valid causal inferences to inform policymaking, research, and practice (Stuart, 2010). This article provides an accessible discussion on the…

Finding a periodic solution to a nonlinear ordinary differential equation is in general a difficult task. Only in a very few cases can direct methods be applied to an equation to find initial values leading to a solution of the corresponding initial value problem that is periodic. Oscillatory periodic solutions have such practical importance that…

Pure and applied mathematical research is discussed briefly. The author gives an example of mathematical research on functional differential equations that was motivated by a student error on a calculus examination. (MK)

Comments on preceding articles include "Studies of the Hexagonal Model" (Holland, Gottfredson); "Structure of Occupations" (Dawis); "Is Holland's Theory Worthy of So Much Attention?" (Schwartz); "Does Enough Evidence Exist to Modify Holland's Theory?" (Hansen); "Research and Application Using Holland's…