– In order to carry out the HTA it is necessary to obtain data on how the task is performed. This could be collected via observation of the task in question or from a detailed specification of the device under analysis. Alternatively, interviews or questionnaires with people that have first-hand experience of performing that task could be conducted to gather the necessary detail. Define the overall task goal, which will be presented as the top level in the HTA. An example might be "increase fan speed by two steps". This describes what is being achieved by performing the task; however, at this stage there is no indication of how the task will be performed. Determine the next level of sub-goals by breaking down the overall goal. A sub-goal for the above example might be "open the climate menu". This provides more information about how to accomplish the task; however, it can still be broken down into smaller units, which will describe the individual operations (performed via the visual, manual or cognitive modes) that need to be performed. Continue breaking down the sub-goals until all operations are identified. Operations in the "reduce fan speed task" will include "move finger to climate menu button" and "touch climate menu button". Define plans to describe how to perform the operations in each sub-goal level of the hierarchy. In the fan speed example, the two operations will have to be performed in series, one after the other. The plan will instruct the user to "perform 1, then 2". Operations can also be performed in parallel, and in this case the plan would instruct the user to "perform 1 and 2 together". Numbers should be assigned to the different levels in the hierarchy. Organising the hierarchy Each level in the HTA should be numbered according to its hierarchical level: The overall goal is the highest hierarchical level and should be numbered 0. The first sub-goal in the hierarchy will be 1, also with plan 1. Further levels just extend this system - third hierarchical level: 1.1, fourth hierarchical level: 1.1.1, and so on. A HTA can be represented in list or diagram form. In list form lines should be indented to denote the different hierarchical levels. In diagram form each operation should be placed within a box and links should be made between them: a lower hierarchical level should branch from underneath a higher level operation.