Textbook of Usability

Throughput (TP) is the standard metric for comparing pointing devices, defined as the index of difficulty of a task divided by the mean movement time:

$$TP = \frac{ID}{MT}$$

where $ID$ is the Fitts's Law index of difficulty (bits) and $MT$ is the movement time. Throughput is measured in bits per second and captures both the speed and accuracy of a device: a device that is fast but inaccurate, or accurate but slow, will have lower throughput than one that is fast and accurate.

ISO 9241-9 specifies throughput as the primary performance metric for pointing device evaluation. Typical values:

• Mouse: 4–5 bits/second
• Touchscreen (direct finger): 5–7 bits/second
• Stylus: 4.5 bits/second
• Trackpad: 3–4 bits/second
• Eye tracking: 3–4 bits/second

The metric allows comparisons across devices and conditions that differ in individual accuracy or speed. A device optimisation that increases speed at the cost of accuracy may leave throughput unchanged — revealing that the speed gain is illusory.

Throughput calculations incorporate "effective" target width based on actual endpoint distribution (not just the nominal target), which adjusts for the speed-accuracy trade-off users choose on a given device.