\section{Evaluation Methods}\label{exampleevaluations}
Appropriate evaluation of the shared control techniques is required to assess their merits and drawbacks. Nuttin et al (2002) and Parikh et al (2005) have listed a number of evaluation methods. Quantitative measures such as time to completion, speed of the wheelchair, number of collisions, number of stops, driver-wheelchair interaction, quality of trajectory, level of fatigue etc. have been widely used by developers to assess their systems. It has also been found that subjective evaluation in the form of questionnaires answered by the users or evaluation by observers, are useful sources of feedback.

Driver-wheelchair interaction can be measured by the number of times a user issues commands through the access device. Zeng et al (2006) conducted tests where volunteers were instructed to minimise the movements of the joystick to assess the effectiveness of their path guidance algorithm implemented in the Collaborative Wheelchair Assistant (CWA).

The five-point Likert scale (1 - strongly agreed, 5 - strongly disagreed)  may be used for answering questionnaires (Carlson \& Demiris, 2010; Petry et al, 2010). Another method of subjective evaluation is using the NASA-TLX workload assessment procedure (Parikh et al, 2005). The NASA Task Load Index (NASA-TLX) is an evaluation procedure to determine the workload of a task performed by the rater (NASA, 1986). Though originally developed more than twenty years ago by the Human Performance Group at NASA Ames Research Centre for the field of aviation, it has been applied for assessing subjective workloads in many other human-machine environments such as tele-operation of robots, automobile driving and military operations (Hart, 2006), as well as laboratory experiments such as memory tasks, grammatical reasoning etc. (NASA, 1986). Six subscales, namely, \textquoteleft{Mental Demand}', \textquoteleft{Physical Demand}', \textquoteleft{Temporal Demand}', \textquoteleft{Performance}', \textquoteleft{Effort}' and \textquoteleft{Frustration}', are rated on a scale of 0 to 100 for each task and the ratings are weighed by their contributions to the workload of the task. Hence the weighted average of the variable ratings provides the overall workload for the task. Moreover the weights of the component subscales may be compared in order to identify the source(s) of workload (Hart, 2006). Further details of the procedure for calculation of workload using the NASA-TLX scale are provided in \hyperref[evaluation]{Chapter 5}.

Parikh et al (2005) and Carlson \& Demiris (2010) have also used secondary tasks to assess the cognitive complexity of wheelchair driving tasks. These tasks are visual, auditory or cognitive tasks such as mental arithmetic problems, which aim to distract the user from the primary task of wheelchair driving. This can be compared to a scenario where the use of a mobile phone while driving a car can lead to accidents due to lack of concentration. Since shared control techniques are designed to handle these kinds of situations, creating these scenarios in the laboratory provides useful insight into the behaviour of both the user and the intelligent wheelchair in these conditions. Parikh et al (2005) used mathematical problems as secondary tasks and evaluation was based on the number of correct responses. Carlson \& Demiris (2010) used a visual task requiring a physical response. The users had to respond to white images flashing on a blue screen at random intervals (between 100ms and 100ms), on one of the four quadrants of a tablet PC screen, by pressing the corresponding button on a joypad. Evaluation was based on the number of correct responses and the time taken to respond to each image.

Suitable evaluation methods should be selected depending on the implementation of the shared control strategies.
