Читать книгу Eye Tracking the User Experience - Aga Bojko - Страница 40

Sampling rate is one of the most important features of an eye tracker. Measured in hertz (Hz), it is the number of times the eye tracker registers the person’s gaze location per second. This means that every second a 120 Hz eye tracker collects 120 data points for each tracked eye. That’s over 200,000 data points recorded during a 30-minute eye tracking session! But don’t worry, you probably won’t have to deal with all of these raw data—they will be nicely combined into meaningful fixations for you by the software.

The range of sampling rates currently available is quite wide—from 25 Hz to 2000 Hz. If you only want to determine where people are looking, a higher sampling rate doesn’t do a better job at identifying gaze location than a lower sampling rate. However, the higher the sampling rate, the more precisely you can measure when a fixation started, and the smaller the error in fixation duration. Fixation durations measured by 25–30 Hz systems can have an error of +/– 20 ms, while durations measured by 250 Hz eye trackers can have an error of only +/– 2 ms.¹

Therefore, if you are interested in measuring not only where people look but also for how long (and as a UX researcher, you might be), you should be aware of this error because it can increase noise in your data. To reduce the error, you should aim for higher sampling rates. Alternatively, you can collect more data (for example, by testing more participants) to wash away the noise.

Considering the fact that systems with higher sampling rates cost more, how high do you really need to go? The general rule of thumb for UX research is 50–120 Hz. These sampling rates will produce a fixation duration error of +/– 10 ms or less. Keeping in mind that a typical fixation lasts between 100 and 500 ms (half a second), an error of this magnitude is generally acceptable in our field.

Sampling rates of 250 Hz and higher are required for research measuring characteristics of saccades (for example, their speed), as well as micro eye movements, which can be of interest to other fields, such as neuroscience.