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The accuracy of an eye tracker is the average difference between what the eye tracker recorded as the gaze position and what the gaze position actually was. You should want this offset to be as small as possible, but it is unrealistic to expect it to be equal to zero.

Accuracy is measured in degrees of visual angle (see Figure 3.6). Typical eye tracking accuracy values fall in a range between 0.5 and 1 degree. To give you an idea of what that means, one degree corresponds to a little less than half an inch (1.3 cm) on a computer monitor viewed at a distance of 27 inches (68.6 cm). In other words, the actual gaze location could be anywhere within a radius of 0.47” from the gaze location recorded with an eye tracker with one degree of accuracy (see Figure 3.7).

FIGURE 3.6 To determine the visual angle of an object, extend lines from the eye to the top and bottom of the object. The farther (or smaller) the object, the smaller the visual angle.

FIGURE 3.7 The “X” indicates the gaze location the eye tracker recorded. Assuming tracking accuracy of 1°, 1680 x 1050 px screen resolution, and 27” viewing distance, the participant’s actual gaze location could have been anywhere inside of the circle.

Accuracy values reported in eye tracker manuals are measured under ideal conditions, which typically include testing participants with no corrective eyewear and taking the measurement immediately after calibration. During “real research,” the difference between the reported and actual gaze locations can be larger for participants wearing glasses or contact lenses, or those who moved at some point following the calibration procedure.

Precision is a measure of how well the eye tracker is able to reliably reproduce a measurement. Ideally, if the eye is in the same location in two successive measurements, the eye tracker should report the two locations as identical. That would be perfect precision. In reality, precision values of currently available eye trackers range from 0.01 to 1 degree. These values are calculated as the root mean square of the distance (in degrees of visual angle) between successive samples. Because the precision values reported by manufacturers are measured using a motionless artificial eye, tracking real eyes will exhibit less precision.

Accuracy and precision are not the same but they are often confused with one another. Table 3.3 should clarify the distinction between the two. The cross in each cell of the table indicates the actual gaze location, while the dots are gaze locations reported by the eye tracker.

TABLE 3.3 THE RELATIONSHIP BETWEEN EYE TRACKING ACCURACY AND PRECISION