Improving Calibration Quality - SR Research EyeLink Portable Duo User Manual

To begin the validation procedure, select the "Validate" button or press the 'V'
key in the Setup screen. The Host PC display will show the gaze position as a
round colored cursor. Note the movements of the cursors, and the change in
relative horizontal position (vergence) following saccades. Once the cursor
appears stable, and close to the target, press the ENTER key or the spacebar to
accept the first fixation. The remaining points are collected automatically or
manually, as in the calibration process.
As each fixation is collected, a cross is used to mark its computed position
relative to the target. The error (in degrees) is printed next to the cross. As with
the calibration procedure, the user can use the Backspace key in the middle of
a validation sequence to redo data collection for the last or last few validation
points collected. After the final fixation is collected, the average and maximum
errors are displayed at the bottom of the screen, and the accuracy is scored.
Each eye is graded separately, using colored messages similar to the calibration
results:
GOOD (green background): Errors are generally acceptable.
FAIR (grey background): Errors are moderate, calibration should be improved.
POOR: (red background): Errors are too high for useful eye tracking.
Note, this categorical scoring of calibration accuracy is a general guideline and
different fields of research may choose to use different cut-offs than what is
reported above. Always adhere to the convention in your field of research rather
than accepting the categorical scoring of calibration accuracy.
Observe the pattern of errors for each target position. If only one target has a
large error, the participant may simply have mis-fixated that point during the
validation, and the validation may be repeated to check this: press 'ESC' to
return to the Setup screen, and 'V' to repeat the validation. If that target still
has a large error, it is likely that the target was mis-fixated during the
calibration. If a systematic pattern of error is seen (i.e. all fixations on the left
side are too low) there is probably a calibration or camera setup problem. In
either case, press 'ESC' to return to the Setup screen, adjust the setup as
needed and re-calibrate, then repeat the validation process.

3.9 Improving Calibration Quality

The quality of the calibration determines how useful the recorded data and how
accurate the gaze calculation will be. Below are some simple procedures to
improve data quality and gaze accuracy:
• Always ask the participant to look at the four corners of the display after
performing the camera setup. Be sure to instruct the participant to fixate
An EyeLink Portable Duo Tutorial: Running an Experiment 61
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