Randomization Tests for Single Case Designs with Rapidly Alternating Conditions: An Analysis of p-Values from Published Experiments.

Abstract

Two common barriers to applying statistical tests to single-case experiments are that single-case data often violate the assumptions of parametric tests and that random assignment is inconsistent with the logic of single-case design. However, in the case of randomization tests applied to single-case experiments with rapidly alternating conditions, neither the statistical assumptions nor the logic of the designs are violated. To examine the utility of randomization tests for single-case data, we collected a sample of published articles including alternating treatments or multielement designs with random or semi-random condition sequences. We extracted data from graphs and used randomization tests to estimate the probability of obtaining results at least as extreme as the results in the experiment by chance alone (i.e., p-value). We compared the distribution of p-values from experimental comparisons that did and did not indicate a functional relation based on visual analysis and evaluated agreement between visual and statistical analysis at several levels of α. Results showed different means, shapes, and spreads for the p-value distributions and substantial agreement between visual and statistical analysis when α = .05, with lower agreement when α was adjusted to preserve family-wise error at .05. Questions remain, however, on the appropriate application and interpretation of randomization tests for single-case designs.