Kempthorne developed a randomization-based approach to the statistical analysis of randomized experiments, which was expounded in pioneering textbooks and articles.
[1] Kempthorne is the founder of the "Iowa school" of experimental design and analysis of variance.
[2] Kempthorne and many of his former doctoral students have often emphasized the use of the randomization distribution under the null hypothesis.
Kempthorne's randomization-analysis has influenced the causal model of Donald Rubin; in turn, Rubin's randomization-based analysis and his work with Rosenbaum on propensity score matching influenced Kempthorne's analysis of covariance.
[3] Oscar Kempthorne was skeptical towards (and often critical of) model-based inference, particularly two influential alternatives: Kempthorne was skeptical of, first, neo-Fisherian statistics, which is inspired by the later writings of Ronald A. Fisher and by the contemporary writings of David R. Cox and John Nelder; neo-Fisherian statistics emphasizes likelihood functions of parameters.