Psychological construct can be linked to others in an explanatory framework from which further predictions are possible and testable. The ‘variables’ that we use in psychological research are observable measures of often unobservable constructs. Psychologists often need some measure of general intelligence but they know that ‘intelligence’ is too broad a concept to be pinned down by a relatively short test. They state instead that, for the purposes of their specific investigation, the test they use will be their operational definition of intelligence. A psychologist might be interested in establishing some quality of all human behaviour, or in the characteristics of a certain group, such as those with strong self-confidence or those who have experienced pre-school education. In US psychology departments the student is often required to participate in a certain number of research projects as part of the departmental ‘subject pool’; the ‘volunteering’ only concerns which particular projects they choose. Some UK universities operate a similar system.