Control parameter
The image of a dial being turned: the control parameter is not the outcome itself, but a condition that makes certain outcomes possible.
A control parameter determines which dynamic pattern a system adopts. It is not itself the pattern; a change in the control parameter can, however, cause one pattern to tip into another when a critical threshold is crossed.
In thermodynamically open systems, the control parameter usually refers to the supply of energy: the degree to which a system is activated or, in psychological terms, motivated. In a more general sense, it influences the interaction between the parts of a system. A small change often has little effect, but beyond a certain point a small further change can bring the system rather suddenly into a new state. A familiar example is water, which remains liquid for a long time as the temperature rises and turns to vapour at one hundred degrees (at sea level). Here the temperature is the control parameter, and the state of matter is the pattern.
Why we phrase it cautiously in mental health care
In psychological systems, unlike in physics, we rarely have direct access to the control parameters. We cannot set them with a dial. For this reason, in the clinical context one often prefers to speak of an order transition rather than a phase transition in the strict sense. The phenomenon appears related, but this phrasing acknowledges that we usually do not have the underlying parameter in hand.
Even so, the concept is illuminating here as well, because it shifts attention from the outcome to the conditions. Factors that might act as control parameters in a treatment, such as safety, meaning, activation and the quality of the working alliance, cannot be steered directly towards a desired end state. They can, however, be influenced in such a way that a reorganisation becomes more likely. In our impression, this is one of the most useful insights that complexity science has to offer practice.