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such as a room. This input information goes to the device that compares the sensed value to the thermostat’s settings. As long as there is no notable difference between values, nothing will happen. If the comparator does detect a difference between values, it sends a message that turns on the heater (output function) which begins to bring warm air into the room. The thermostat will continue to request activity from the heater until the room has warmed up enough so that it can no longer sense a discrepancy between the current air temperature and the thermostat’s setting. In Section 2 of this chapter we will provide examples of feedback processes in health informatics (HI) interventions. As Figure 1 shows, effects on a system’s environment do not only depend on operations by the output function. Disturbance, originating outside the loop, does not affect the components of feedback loop directly, but it can modify the agent’s perceptions via the input value and lead to changes in the discrepancy from the standard. These changes can be adverse (creating or increasing the discrepancy) or favourable (closing the discrepancy). In the example of the thermostat, an open window on a cold day might allow cold wind to enter the room and reduce room temperature; increasing the gap with the thermostat’s target temperature range. Alternatively, a warm sun shining through the window or a large group of people standing in the room radiating excess body heat might establish the opposite effect. In that case, the result of the disturbance is that there is no need for an output adjustment because the system observes no discrepancy. Hence the main purpose of the feedback loop is not to undertake an action, but to create and maintain the perception of a specific desired condition i.e. no discrepancy between the input and reference value. 1.2. Hierarchical systems and reference values Feedback loops are often organised in a hierarchical fashion such that there are superordinate and subordinate systems (Figure 2) [4]. Each system relates to superordinate (at the higher end of the hierarchy) or subordinate (at the lower end of the hierarchy) goals, where achievement of subordinate goals is a requisite for attaining superordinate goals. Superordinate systems act by changing the reference value of the subordinate system at the lower level in the hierarchy. That is, the output of the superordinate level sets the standards for the next lower level. In turn, the subordinate system changes the reference value for the next lower level, and so on. Building on the thermostat example, the thermostat receives its reference value from a superordinate system which may be a person in the room. This person also has a reference value e.g. ‘be comfortably warm’. Rather than operate directly on the environment to produce heat, for example, by building a fire, the person operates by providing a new reference value to the subordinate system–resetting the thermostat to a warmer temperature setting. Due to the change in reference value, the thermostat activates the furnace and the room temperature rises. Hierarchically-organised systems each act to create their desired condition and monitor their own input which exists at their own level of abstraction. In the example of the two-level hierarchy of the person and their thermostat, the thermostat assesses air temperature whereas the person assesses their comfort level. Superordinate goals at the higher end of the hierarchy tend to be more abstract whereas subordinate goals at the lower end are more tangible and concrete. As the subordinate system executes, both systems progress towards discrepancy reduction. Higher level (i.e. abstract) goals may be achieved more gradually over time than lower level (i.e. concrete) goals. W.T.GudeandN.Peek /ControlTheory toDesignandEvaluateAuditandFeedback Interventions 161