The true score is the actual score that would reliably reflect the measurement (e.g., for a person) on a given construct (e.g., a score of 76 out of 100 in an IQ test actually reflects the intelligence of the person taking the test if that person took another IQ test the next day, we would expect them to get 76 out of 100 again, assuming that we are only seeing that person's true score and not any error). Therefore, the score measured (e.g., 0-100 in an exam) for a given variable consists of the true score plus error. However, the measurement procedure that is used to measure a variable introduces some amount/degree of error, whether small or large. Some variables are more stable (constant) than others that is, some change significantly, whilst others are reasonably constant.

So what do we mean when we say that a measurement procedure is constant or stable? In principal, a measurement procedure that is stable or constant should produce the same (or nearly the same) results if the same individuals and conditions are used. This assumption, that the variable you are measuring is stable or constant, is central to the concept of reliability. When we think about the reliability of these variables, we want to know how stable or constant they are. In quantitative research, the measurement procedure consists of variables whether a single variable or a number of variables that may make up a construct. Since there are many ways of thinking about intelligence (e.g., IQ, emotional intelligence, etc.), this can make it difficult to come up with a measurement procedure that has strong validity. For example, if we want to measure the construct, intelligence, we need to have a measurement procedure that accurately measures a person's intelligence. This measurement procedure should provide an accurate representation of the construct it is measuring if it is to be considered valid. These questionnaire items or interview questions are part of the measurement procedure. For example, we may choose to use questionnaire items, interview questions, and so forth. When we examine a construct in a study, we choose one of a number of possible ways to measure that construct.

Example of reliability in research