The research about which variables have the strongest predictive value for success in the workplace is unequivocal; general intelligence, or general mental ability, ranks as the single most important predictor. This, in descending order of importance, is followed by a properly conducted structured interview, work sample tests, and conscientiousness and other aspects of personality. Predictive validity is increased by combining these variables.
The importance of general intelligence in job performance is related to complexity. Occupations differ considerably in the complexity of their demands, and as that complexity rises, higher general intelligence levels become a bigger asset and lower levels a bigger handicap.
The economic outcome from recruitment and staff selection can be significant, with even marginal differences in rates of return yielding big gains – or losses – over time. Hence, even small differences in general intelligence among people can exert large, cumulative influences across social and economic life.
Conversely, the cost of a bad hire is estimated to be up to a year’s salary depending on the role. That’s how much it adds up to by the time you recruit them, on-board and orient them, train them and have them in a position where they can perform effectively.
If you decide to incorporate any psychometric testing into your recruitment process I would recommend you start with a good test of general intelligence. Fermion recommends the Reynolds Adaptable Intelligence Test (RAIT) or the brief version for screening purposes, the Test of General Reasoning Ability (TOGRA).