A requirement to do the work personally is an element of control because it indicates that the employer is interested in the methods as well as the results. The employer is interested not only in getting a desired result, but also in who does the job.

When services must be performed by the worker and the worker does not usually have the ability to assign his or her work to other workers, an employer-employee relationship is assumed.

An independent contractor may perform services personally, but is capable of hiring someone else to perform the work. An independent contractor may assign the work to others. The right to substitute another worker's services without the employer's knowledge suggests the existence of independent contractor status. 

Social Security Handbook, Chapter 807: Do you personally perform your job functions?