Cattell 

Psychometrics involves the measurement and representation of psychological variables (such as intelligence, aptitude or personality type). It is heavily based on statistics and mathematical analysis. Measurement of individual differences is done using tests designed to be highly reliable (i.e. giving consistent results) and valid (i.e. measuring what they are supposed to measure). With respect to the study of personality, the psychometric perspective measures personality, describes personality structure, and often tries to explain the origins of personality in terms of biology, asking whether it is individual differences in biology that cause individual differences in personality. Although approaches such as humanistic psychology are also particularly interested in individual differences, the underlying philosophy of the two approaches is totally different, and form an interesting contrast. Humanistic psychologists attempt to take an 'idiographic' approach, that is try to understand a person in terms of their own, unique, worldview; this tradition also usually focuses on qualitative data, and takes a holistic view of people. Psychometrics, in contrast, will focus on quantitative data, using categories applicable to everyone, devised by the psychologist, into which the 'individual differences' of the person examined must fit. The focus is on aspects of people – particular dimensions of their behaviour and feelings; the concern is not with 'whole people' and their inner experience; the aim is to make statements about people in general. These are often based on 'personality traits': adjectives that describe enduring characterisics of people, used as the basic 'building blocks' of theories about personality. In attempting to measure personality, psychometrics focuses on the ways in which humans are like each other, in terms of their positions on broad dimensions, rather than with the ways in which each person is unique. The psychometric tradition has also typically seen human beings as having relatively fixed personality traits, in contrast to the humanistic emphasis on possibilities for self-directed change and transformation. Psychometrics has a long tradition in psychology, going back to Galton (around 1884) and is usually associated with biologically-based theories of evolution and heritability. This association led to (in modern terms) some rather ethically dubious connections between psychometrics at that time and movements such as eugenics, the desire to improve humanity through 'selective breeding'. In judging the viewpoints of earlier generations we perhaps do need to take into account the changing moral climate produced by changing socio-political contexts – e.g. eugenics, post-Hitler, probably has quite different connotations to those it would have had in the nineteenth century. As a tradition, psychometrics and individual differences psychology – whether in relation to personality, intelligence or other aspects of psychological measurement – has tended to develop and use its methods for practical applications as well as pure research. Psychometric instruments play an important role in occupational psychology, i.e. psychology applied to a work setting. The use of psychometrics to examine individual differences has been a crucial part of the growth of psychology as an empirical and scientific discipline. Over the last century, at first driven by education policies, and then recruitment into the military in the Second World War, increasingly sophisticated psychometric techniques have helped to develop a wide variety of psychological tests and led to a highly profitable industry. There are now many established tests of aptitude, intelligence and personality which are used both for research and in applied settings such as education, occupational testing for job selection, career counselling and in forensic psychology and clinical practice. 

Personality structure and Personality development . The term 'personality' can have widely divergent meanings. It can be seen as the outcome of the factors that make a person different from others in their patterns of thought, values, beliefs, emotions, and social roles and relationships. But personality can also be seen as the 'something'- the underlying construct- that leads to people's consistency in the way they behave and what they feel, over time and across many situations. In this sense, personality may be essentially unique to each of us but it can also be seen as a profile of underlying dimensions (possibly with a biological bases) that are common to many people.\nPersonality is an important concept in clinical, psychoanalytical and psychiatric contexts, particularly in terms of diagnosing and treating people seen as having disorders of the personality (such as compulsive, psychopathic, or paranoid personality disorders). 

Psychometric (assessment and techniques). Psychometric methods involve measuring psychological characteristics, such as personality, intelligence or aptitudes. Statistical techniques are particularly important, to establish reliability (does the test produce consistent results?) and validity (is the test measuring what it is supposed to be measuring?). Another important concept is 'norms'. For example, on intelligence tests, the norm for children of a particular age is the average score gained by the children of that age on whom the test was standardised. It therefore reflects typical development for that age group.