According to Harmony Science Academy, a controlled experiment is one in which a hypothesis is tested by the manipulation of a single variable. The change that results from the manipulated variable establishes causality through directional cause and effect.
The component of an experiment that is subject to manipulation is called the independent variable. In a controlled experiment, a researcher would observe the response of the component that remains unchanged, referred to as the dependent variable, when manipulation occurs. For example, if it is hypothesized that children are more cooperative when they are rewarded, the researcher would first observe levels of cooperation without a reward and then note the effects once a reward was introduced. If the children become more cooperative, this supports the researcher's hypothesis. This is so because causality is observable once the independent variable is introduced.