Job evaluation is the process of trying to define a hierarchy of jobs in an organization. It seeks to spell out which jobs are more important to the firm and which are less important.
Job evaluation can be a science because it can rest on some objective factors. When doing a job evaluation, one can look at such things as the relative amount of revenue produced by each job. This is an objective measurement just as scientific measurements are objective.
But not all aspects of the value of a job are quantifiable. Some jobs may support other jobs that bring in the money. It is hard to quantify how much those support jobs are worth relative to the jobs that do clearly bring in money. Therefore, there is an element of art involved in job analysis. There is a degree to which a person must try to define the value of a job in ways that cannot be easily quantified.