Abstract: Apparatuses and methods for displaying a blood pressure monitoring instrument reading along with inaccuracy values that vary over apparatus life and according to current operating conditions are disclosed. These inaccuracy values account for variability under different operating environments, over time and number of device measurement cycles, and across the range of possible instrument reading conditions. Determination of inaccuracy values may be based both on apparatus history and measurement under the current operating condition. Methods can further include informing a user of confidence in the instrument's current measurements based on the instrument's measurements and inaccuracy.