\section{Discussion of Experimental Procedure}\label{disExpProc}

Testing was limited to a population of only 5 volunteers due to time and resource constraints and it was not possible to ascertain if the difference in mean workload scores for the various tasks are statistically significant. A more fruitful evaluation can be achieved with a larger subject pool of different age groups consisting of able-bodied volunteers and people with disabilities.

In the tests conducted for Obstacle Avoidance with secondary task, it was observed in the case of three volunteers that the wheelchair had been brought to a halt in front of an obstacle, waiting for their commands to proceed. Two inferences may be drawn from these observations. Firstly, since all three of them had no previous gaming experience it could be deduced that the benefits of shared control may be more visible for users with poorer driving skills and that there could be a correlation between gaming experience and wheelchair driving skills.  This impact could be studied using a larger population with varied gaming practice. The second inference relates to the effectiveness of the secondary task in simulating a condition of reduced ability because while all the three volunteers were able to manoeuvre the wheelchair in manual mode, their skills were suppressed in the presence of the secondary task.

The volunteers' responses to the secondary task were also interesting to observe. One of the volunteers skipped slides to concentrate on his driving. Most of them gave wrong answers occasionally in the attempt to react as fast as possible, as was observed by Carlson \& Demiris (2010). The design of the secondary task may be improved by studying the variability in volunteers' responses and devising a method suitable for testing across different population groups or using different sets of secondary tasks for different people depending on their abilities.
In the tests conducted, between-rater variability was observed due to different interpretations of the NASA-TLX subscale definitions. In future tests, explicit instructions may be provided to the volunteers, citing examples of sources of workload within the tasks, to reduce the variability. For example, pressing the keypad can be identified as a physical task which would contribute towards the physical demand subscale. This would help to determine the magnitude of user interaction from the ratings of that subscale.

While testing Obstacle Avoidance mode, the rating of the tasks using the NASA-TLX scales was conducted immediately after the volunteer navigated the obstacle course containing the columns and narrow spaces, which were the main sources of frustration in the whole circuit. As this could have influenced their rating and choice of sources of workload, it is proposed that separate tests be conducted for different difficulty levels.

The Obstacle Avoidance tasks were conducted in the Training Course VE, which was created to test the mechanical functioning of the wheelchair such as its ability to negotiate ramps and corners, to verify the properties of inertia etc. It is suggested that testing be conducted in a map created to test various aspects of the shared control behaviours. The environment may contain obstacles of various sizes, passages of different widths etc. and arranged such that tasks with different difficulty levels can be tested separately.
