Measuring Motion SicknessJuly 31, 2014
Many of the recommendations in the Oculus VR Best Practices Guide deal with motion sickness. When developing a game, it’s important to be rigorous in measuring how people feel while playing the game.
Recognize that you are not your user. As you develop a game, you will become more accustomed to the simulator and your game will not feel as jarring, even if it would make an average player sick. You also have experience with video games and jarring images in general that may make you less prone to sickness. Because you yourself are biased, you must measure user sickness while playing the game and treat reports of queasiness appropriately.
So how do you actually measure nausea? We are concerned specifically with simulator sickness, which is less severe than motion sickness for most people, Simulator sickness can also be stopped at any moment if a user closes their eyes or takes off the headset.
The US Navy developed a sickness measurement for pilots that allowed them to measure acclimation to flight simulator sickness across its fleet of simulators, as pilots took increasing numbers of flights. In 1993, Kennedy, Lane, Berbaum, and Lilienthal published the Simulator Sickness Questionnaire (SSQ), a widely acknowledged standard for measuring sickness in simulated environments.
The SSQ is administered immediately after a flight, and asks pilots to rate how much each of 16 symptoms is affecting them, choosing one option of None, Slight, Moderate, or Severe. These symptoms are combined into a score for three different categories, Nausea, Oculomotor (affecting vision), and Disorientation (dizziness, vertigo, etc.). You can view the aggregation and scoring system here.
But you can’t administer the SSQ while you are wearing a headset, flying a simulator, etc - it takes too long to ask 16 questions every N minutes. Keshavarz and Hecht (2011) developed a tool that can be used while the simulator is running - the Fast Motion Sickness Score, or FMS for short. Simply, you ask the subject to rate their level of motion sickness, on a scale from 0 (no sickness at all) to 20 (frank sickness), once per minute. The highest recorded score, as well as the last score, were well correlated (p of 0.79) with the SSQ, and much easier to administer. It’s useful to note that reported sickness generally increased as the study went on, with no decrease, for a period of eighteen minutes.
The Navy published SSQ scores for its fighter simulations, which you can find in the attached PDF. 40% of the population (of Navy fighter pilots, mind you, not the average VR user) experienced no simulator sickness under any conditions. A person at the 65th percentile rated a 7.5 Total Score, at the 75th percentile rated a 15 Total Score, and at the 90th percentile rated a 30 Total Score.
We don’t have corresponding data for virtual reality applications, but it won’t take much to establish baselines. A good practice may be to have users rate their motion sickness level while playing your application, as well as rate users against some baseline application, say, the living room demo application. These numbers will help determine whether some people will get sick no matter what, or whether sickness is specific to your game.
We will cover the causes of simulator sickness and more statistics about motion sickness in another post.