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.