For providing a theoretical structure to the Strong, in 1974, John Holland’s psychology-based codes were incorporated into the instrument. Holland’s theory is based on four main assumptions:

1. Most people can be categorised into six themes and each person may be characterised by one theme or some combination.
2. Job environments can be divided into these same six Themes and each environment is dominated by a particular type of person. Thus, the personality types of co-workers, as much as job requirements, establish the working tenor of a given occupation.
3. People search for environments that let them exercise their skills and abilities, express their attitudes and values, take on problems and roles they find stimulating and satisfying and avoid chores or responsibilities they find distasteful or formidable.
4. Behaviour is determined by an interaction between a person’s personality and the characteristics of his or her working environment. Factors such as job performance, satisfaction and stability are influenced by this interaction.

As shown below, Holland’s theory organises the six Themes by placing them at the six points of a hexagon with those presumed to be the most closely related located adjacent to each other and those most dissimilar located across the hexagon from each other. The order in which they fall around the hexagon is frequently called the R-I-A-S-E-C order. The Strong is the only empirically derived RIASEC instrument.