Paper variability influences the methods used to test paper and the interpretation of the tests results. The composite and viscoelastic nature of paper causes variability, as do moisture and humidity. Also discussed are formation and furnish variability, the anisotropic nature of paper, network theories, hysteresis and preconditioning influences. Interpretation of test results is discussed with emphasis on the fold endurance test: its lack of precision, the wide variation in test results and the possibilities for multiple interpretations. Graphs, diagrams, references.