This topic tests your understanding of the ways in which a measurement device may not accurately reflect the actual physiological situation.

Accuracy

The ability of a measurement device to match the actual value of the quantity being measured.

Precision

The reproducibility of repeated measurements and a measure of their likely spread.

In the analogy of firing arrows at a target, the accuracy would represent how close the arrow was to the bullseye, whereas the precision would be a measure of how tightly packed together a cluster of arrows were once they had all been fired.

Drift

A fixed deviation from the true value at all points in the measured range.

Hysteresis

The phenomenon by which a measurement varies from the input value by different degrees depending on whether the input variable is increasing or decreasing in magnitude at that moment in time.

Non-linearity

The absence of a true linear relationship between the input value and the measured value.

Zeroing and calibration

Zeroing a display removes any fixed drift and allows the accuracy of the measuring system to be improved. If all points are offset by ‘+ x’, zeroing simply subtracts ‘x’ from all the display values to bring them back to the input value. Calibration is used to check for linearity over a given range by taking known set points and checking that they all display a measured value that lies on the ideal straight line.