Challenges in estimating reproducibility of imaging modalities.

Estimating reproducibility is often wrongly thought of as basic science. Although it has a significant clinical relevance, its importance is underestimated. It was Alexander Pope in 1732 who was first to understand the value of reproducibility, with his famous comment "Who shall decide when doctors disagree?". Pope's question concerns the medical doctors' opinion on a patient's status, which from a statistical point of view may be considered a categorical variable. However, the same question may be posed for continuous quantitative variables. Reproducibility is complementary to variability: the larger the variability, the lower the reproducibility, and vice versa. Thus, we can think at them as interchangeable, even thought statistical methods have been developed for the estimation of variability. The question now is "Why do we need to know the reproducibility of measurements? ". The most important and simplest answer is that we need to know how reliable a measured value or a subjective judgment is before taking clinical decisions based on this measurement/judgment. Integrating this knowledge in clinical practice is a key aspect of evidence-based medicine.