They are also called primary mental abilities (PMA). The seven primary group factors include: verbal comprehension "that can be measured through vocabulary tests; word fluency "tests that measure the ability to generate words beginning with a specified letter or that rhyme with another word; number "the speed and accuracy of arithmetic calculations; space "measured by tests entailing mental rotation of objects; associative memory "measured by picture and word-recall tests; perceptual speed "such as checking for similarities and differences in visual details and reasoning "measured by tests such as number series, arithmetic reasoning and analogies (Sternberg, 2006). .
Gulford proposed what he called the Structure of Intellect, or SI model. This model describes and classifies intellectual abilities along three basic dimensions: operations "what the individual does, the type of mental operation performed; contents "the kind of information on which the operation is performed and products "the form in which the information must be conceptualised by the subject. These are divided into: five types of operations (cognition, memory, convergent production, divergent production and evaluation); four types of contents (figural, symbolic, semantic and behavioural) and six types of products (unit, classes, relations, systems, transformation and implications). Once these are organised into a three-dimensional classification system, they result in 120 separate mental abilities, each symbolizing the intersection of one operation, one content and one product. For instance, a vocabulary test would entail the operation called cognition, performed on semantic information with the product being the unit of information (Walsh & Betz, 1995). .
This model led to the inference of many new abilities, such as social intelligence. There are many circumstances where the measurement of definite abilities are essential.