Types of Variable > Manipulated Variable
What is a Manipulated Variable?
A manipulated variable is the independent variable in an experiment. It’s called “manipulated” because it’s the one you can change. In other words, you can decide ahead of time to increase it or decrease it. In an experiment you should only have one manipulated variable at a time.
An experiment generally has three variables:
- The manipulated or independent variable is the one that you control.
- The controlled variable is the one that you keep constant.
- The responding variable or variables is what happens as a result of the experiment (i.e. it’s the output variable).
Example: You want to find out what the effect is of changing lesson lengths is on student exam performance. The manipulated variable would be the lesson times, as that’s what you’re going to change. Controlled variables would be things like ensuring the exam times are the same, or that the students aren’t hungry (perhaps they should all be tested at the same time). The responding variable is exam success, measured by actual scores on exams.
Manipulated Variables in Process Control
In process control there are two types of input variables: manipulated variables and disturbance variables. In this context, the manipulated variable is the input that is controlled by the process operator or control system. The manipulated variables are adjusted by the process operator (or control system) to keep the controlled variables in the system at constant settings. The disturbance variable is a second type of input, and it affects the process outputs. However, unlike manipulated variables they can’t be adjusted by the control system.
As a simple example, let’s say you wanted to keep speed constant in a car. External factors like acceleration, friction or tire pressure cause it to change. An accelerator (gas pedal) can keep the speed constant — figuring out when to use the accelerator is the control. The state of the accelerator is the manipulated variable. The process is speeding up, via the accelerator and the speed itself is the controlled variable.