Another central systems idea is that of feedback, whether negative (self-correcting) or positive (self-reinforcing). Feedback is what allows systems to adapt to the environment and also what allows people to learn. However, although it is relatively simple to engineer predictable feedback mechanisms in mechanical systems, the more complex the system and the greater the number of interdependencies, the more likely that acting on a part of it will generate feedback loops that have unexpected and often perverse consequences in a different part of the system. So, we can predict that optimizing an NHS hospital trust for waiting times and other partial performance measures will have consequences somewhere else in the system. Sure enough, waiting time results have improved, but not the system as a whole, so that hospitals have met their explicit targets, but at the cost of going into the red