Once they are hired, resources and funds are spent on them to extract the best out of them as efficiently as possible. Therefore by defining the KSAs more broadly, the qualities of a candidate can be set apart from the others which make the recruiting procedure easier and more beneficial. 
The knowledge statement makes adequate performance on the job possible while the skill statements refer to the proficient manual, verbal or mental manipulation of data or things. On the other hand the ability statements refer to the power to perform an observable activity at the present time. Integrating these statements can serve as a basis for distinguishing the 'qualified candidates' from the 'unqualified candidates' for a position.  
The development and selection of job performance tools depends upon the type of job. Not all kinds of performance instruments can be attributed to a person and therefore different tools such as Monitoring, Testing, Observations etc are employed. Because competence is multi-dimensional and individual assessment approaches have limitations, it is unlikely that a single approach to assessment will be adequate. 
The primary question with any personnel instrument is overall effectiveness. This is considered an extremely important factor in selecting job performance instrument. ...
required to implement the assessment, equipment or technology required, number of assessments required per examinee, financial cost, and the extent to which an assessment has been used. If an instrument that is not practical to be tested upon the candidates, then it is useless and therefore it must be rejected. Similarly if the time and cost incurred to train and test the candidate is more than the value that can be derived from assessing the instrument then again the instrument it useless. Also the assessment should provide new and useful information that facilitates teaching and learning. For instance, the assessment should allow the collection of enough detailed information that it is possible to know what performance improvements or curricular modifications are needed. If this is not provided by the performance instrument then again the instrument must be rejected. 
Another factor that is equally important to effectiveness is the validity of the data. Valid data provide accurate information about what is being assessed. Different types of evidence may be used to infer validity. It may be inferred when assessment results help to predict performance in actual practice. Validity may be inferred also when it is possible to detect the responsiveness. Data that is invalid is of no use to the company and hence an instrument that extracts invalid data must not be considered for measuring job performance. 
Apart from the factors mentioned above, job performance instruments must be broad so that they cater to various aspects of an employee. This helps in diminishing any bias that may arise in the instruments. 
Furthermore the performance instruments must be developed and selected in an audience oriented fashion that is they must be in accordance with the