Accuracy and precision in the evaluation of computer assisted surgical systems. A definition]

Accuracy represents the outstanding criterion for navigation systems. Surgeons have noticed a great discrepancy between the values from the literature and system specifications on one hand, and intraoperative accuracy on the other. A unitary understanding for the term accuracy does not exist in clinical practice. Furthermore, an incorrect equality for the terms precision and accuracy can be found in the literature. On top of this, clinical accuracy differs from mechanical (technical) accuracy. From a clinical point of view, we had to deal with remarkably many different terms all describing accuracy. This study has the goals of: 1. Defining "accuracy" and related terms, 2. Differentiating between "precision" and "accuracy", 3. Deriving the term "surgical accuracy", 4. Recommending use of the the term "surgical accuracy" for a navigation system. To a great extent, definitions were applied from the International Standardisation Organisation-ISO and the norm from the Deutsches Institut fur Normung e.V.-DIN (the German Institute for Standardization). For defining surgical accuracy, the terms reference value, expectation, accuracy and precision are of major interest. Surgical accuracy should indicate the maximum values for the deviation between test results and the reference value (true value) A(max), and additionally indicate precision P-surg. As a basis for measurements, a Standardized technical model was used. Coordinates of the model were acquired by CT. To determine statistically and reality relevant results for head surgery, 50 measurements with an accuracy of 50,75, 100 and 150 mm from the centre of the registration geometry are adequate. In the future, we recommend labeling the system's overall performance with the following specifications: maximum accuracy deviation A(max), precision P and information on the measurement method. This could be displayed on a seal of quality.