29 CFR Part 1630, Regulations to Implement the Equal Employment Provisions of the ADA (Title I Regulations with amendments issued through May 2016)

(3) Evidence of whether a particular function is essential includes, but is not limited to:

(i) The employer's judgment as to which functions are essential;

(ii) Written job descriptions prepared before advertising or interviewing applicants for the job;

(iii) The amount of time spent on the job performing the function;

(iv) The consequences of not requiring the incumbent to perform the function;

(v) The terms of a collective bargaining agreement;

(vi) The work experience of past incumbents in the job; and/or

(vii) The current work experience of incumbents in similar jobs.