\section{NASA Task Load Index}\label{nasatlx}

The NASA Task Load Index (NASA-TLX), which was introduced in \hyperref[exampleevaluations]{Section 2.4}, has been created with the assumption that the variables, Mental Demand, Physical Demand, Temporal Demand, Performance, Effort and Frustration, in one or another combination, account for the workload of most tasks performed by humans (Hart, 2006). This procedure was hence chosen to compare the workloads related with different wheelchair driving tasks. The influence of the individual subscales on the overall workload would also prove useful in assessing the shared control strategies and help in its improvisation.

The NASA-TLX scale involves measurement of the magnitude of the load, i.e., rating, as well as determining the sources of the load, i.e., weights (NASA, 1986). This two-step procedure ensures that the assessment is insensitive to the variability between different tasks and raters. Between-rater variability could be attributed to differences in the interpretation of the subscale definitions by different people as well as variations in their perception of the major factors contributing to the workload of a task. For instance, while some may consider performance to be the main criteria in gauging workload, others may find mental challenge to be the determiner of workload, irrespective of their own performance in the task. The evaluation of ratings and weights are described below.

\textbf{\textit{Magnitude of Workload Evaluation}}: The rater is provided with a \textquoteleft{Rating Sheet}' (see \hyperref[ratingSheet]{Appendix B.3}) for each task performed by him or her. Each of the six subscales are to be rated on a scale consisting of a 12cm line representing a range of 0 to 100. It is divided by vertical tick marks into 20 equal segments.  For all the subscales with the exception of Performance, 0 corresponds to \textquoteleft{Low}' and 100 to \textquoteleft{High}'. For the Performance subscale, \textquoteleft{Good}' is represented by 0 and \textquoteleft{Poor}' by 100. By placing a mark at the appropriate level on the scale, the user thus provides a numerical rating of that subscale, which is referred to as the \textquoteleft{Raw Rating}'.

\textbf{\textit{Sources of Workload Evaluation}}: Pair-wise combinations of all the subscales (e.g. Mental Demand or Effort, Performance or Frustration etc.) are provided to the user on separate cards arranged in random order for every task or set of similar tasks for which the sources of workload are the same. For each pair, the rater selects the subscale that contributed more to the workload of the task(s). Hence the maximum weight of a subscale is 5, signifying that it is the most important factor creating the workload,  and the minimum 0 implying that it is irrelevant to workload.

While there are no right or wrong answers in marking these scales, the raters are asked to be consistent in their reasoning throughout the experiments in order to produce meaningful results. 

\textbf{\textit{Calculation of Overall Workload}}: The Sources-of-Workload Weights given to the subscales by each rater for a task are calculated by summing the number of times a particular subscale was selected by him or her. The Raw Rating of each subscale is then multiplied by its Weight to produce the \textquoteleft{Adjusted Ratings}' for that subscale. The overall or mean weighted workload equals the ratio of the sum of the Adjusted Ratings for all the subscales and 15, i.e., the total number of weights. Hart and Staveland (1988) have provided an example of the calculation of overall workload from the subscales ratings and weights.