Abstract: An estimate of random error in an eye-tracking system is done directly from eye-tracker outputs during a trial, without the need for an explicit calibration process. The distances traveled between adjacent user observations are computed, and the random error ?r of the eye-tracker system is estimated using the statistical distribution of the computed distances. In the preferred embodiment, the distances traveled between adjacent observations is sampled on a continuous basis. The process includes measuring the mode (peak value) of the distribution in observation distances. These values are sorted by increasing distance, and a window of about 50 observations is used to estimate the mode of the distance distribution. A running estimate of the mode is computed, and the result is divided by a constant. A preferred constant of 1.61 was derived using a series of Monte Carlo simulations.