资料题：Methods of additional payments are adopted for indirect workers because（　）。

As is known to all, the organization and management of wages and salaries are very complex.Generally speaking, the Accounts Department is responsible for calculations of pay, while the Personnel Department is interested in discussions with the employees about pay.If a firm wants to adopt a new wage and salary structure, it is essential that the firm should decide on a method of job evaluation and ways of measuring the performance of its employees.In order to be successful, that new pay structure will need agreement between Trade Unions and employers.In job evaluation, all of the requirements of each job are defined in a detailed job description.Each of those requirements is given a value, usually in"points",which are added together to give a total value for the job.For middle and higher management, a special method is used to evaluate managers on their knowledge of the job, their responsibility, and their ability to solve problems.Because of the difficulty in measuring management work, however, job grades for managers are often decided without reference to an evaluation system based on points.In attempting to design a pay system, the Personnel Department should compare the value of each job with those in the job market.It should also consider economic factors such as the cost of living and the labour supply.It is necessary that payment for a job should vary with any differences in the way that the job is performed.Where it is simple to measure the work done, as in the works done with the hands, monetary encouragement schemes are often chosen, for indirect workers, where measurement is difficult, methods of additional payments are employed.