As defined in the Oxford English Dictionary intelligence is “the faculty of understanding; intellect. Also as a count noun: a mental manifestation of this faculty, a capacity to understand”. Intelligence is widely accepted to be mostly an innate trait that expresses itself by the ability to acquire and apply knowledge and skills. In psychometric terms we would say that intelligence is a latent construct, a factor that cannot be directly measured or observed and needs to be indirectly assessed by means of indicators. Examples of tests used to measure intelligence are the Stanford-Binet intelligence scales, standardised IQ tests, the Wechsler Adult Intelligence Scale (WAIS) or the Wechsler Child Intelligence Scale (WCIS).
As is the case with most latent construct the measurement of intelligence tends to be norm referenced (i.e. the individual is compared to a reference group or population).
Intelligence is a highly complex construct and presently it is acknowledge that it includes at least four major components: verbal comprehension, perceptual reasoning, working memory and processing speed.
As we move away from elitist and racist views that were commonly held during the 19th century of intelligence entitling a group of people to social prominence and dominance, IQ and other intelligence measuring tests have been falling out of favour and replaced by knowledge and skills based tests-the sole exceptions being tests that allow for the identification of special needs and cognitive impairments in order to offer support and tailored educational approaches to individuals.

Knowledge is one of the possible expressions of intelligence and can be measured directly. Individuals are assessed on the mastery of corpora of information, the mastery of skills or their ability to perform tasks and solve problems. The majority of tests sat throughout someone’s educational journey from primary to graduate education tend to be knowledge tests- SATS and international large scale assessments like PISA are examples of knowledge tests, but we could also include drivers test as an example. Unlike intelligence tests many knowledge tests are criterion based (i.e. there’s a clear and objective specification of the tasks to be accomplished and information bits that need to be provided as responses to the test items and a mensurable level of mastery that needs to be displayed).
Knowledge tests are useful both as diagnosis as well as evaluation tools to identify if individuals are able to operate at a minimum level in order to meet the demands and requirements of specific activities (e.g. driving a car without constituting a risk to themselves or others) are be able join the labour market or proceed to post-graduate studies or even being empowered to have agency over their own interaction with society (e.g. minimum levels of literacy to be able to read a newspaper).

As an example of an intelligence test we can mention the Wechsler Adult Intelligence Scale WAIS-IV. WAIS-IV was released in 2008 by Pearson and is currently the most widely used IQ test for adults and adolescents worldwide. It’s quite a lengthy assessment composed of 10 tests and 5 optional sub-tests that evaluate intelligence over four distinct but correlated subscales: verbal comprehension, perceptual reasoning, working memory and processing speed. As strengths of the WAIS IV we can quote its comprehensiveness in evaluating multiple intelligence domains, standardised administration procedures that ensure reliability and comparability, diagnosis utility and the inclusion of normative data that has been validated in a number of studies and ensures the results are relevant and reflective of current population demographics and cognitive trends. The disadvantages of this test are mostly related to its length and complexity (which can lead to test-taker fatigue) and the requirement that is administered by highly specialised practitioners under strictly controlled conditions. There is also some evidence of cultural bias favouring specific subgroups and lack of representativity of the population sample used to derive normative information (mostly based on the adult population of the USA and Canada).