The scores that are obtained in a research study are the result of observing and measuring variables. Some variables are, such as height, weight, and eye colour are well-defined, concrete identities that can be measured and observed directly. On the other hand, many variables studied by behavioural scientists are internal characteristics that cannot be observed or measured directly. However, we all assume that these variables exist and we use them to help describe and explain behaviour. Variables like intelligence, anxiety, and hunger are called constructs, and because they are intangible and cannot be directly observed, they are often called hypothetical constructs. Although constructs are internal characteristics that cannot be directly observed, it is possible to observe and measure behaviours that are representative of the construct. The external behaviour can be used to create an operational definition for the construct. An operational definition measures and defines a construct in terms of external behaviours.