Your results may vary: the imprecision of medical measurements

[...]most importantly, there is biological variation. Biological variation comes about through the convergence of the innumerable perturbations of our physiology, state of health, living environment, diet, activity, stress, mood, the weather and climate, and environmental exposures. The first data column of the table conveys the actual analytical confidence interval or error due to the measurement process alone, without any consideration of biological effects. [...]if a significance level larger than 0.05 is chosen, the required change will be smaller (see appendix).