According to the EEOC, "essential functions are the basic job duties that an employee must be able to perform, with or without a reasonable accommodation", in order to be qualified for a job. It is important to remember that in order to be protected under the ADA, an individual must be qualified. Therefore, essential functions are a key component in determining whether or not someone is qualified for their job.
Essential job functions are determined using a number of factors. The EEOC states that "if a written job description has been prepared in advance of advertising or interviewing applicants for a job, this will be considered as evidence, although not conclusive evidence, or the essential functions of a job." Other considerations in determining essential functions of a job include whether the position exists to perform that function, the number of employees available to perform the function or among whom the function can be distributed, and the degree of expertise or skill required to perform the function. The EEOC also considers the employer̥s judgement on what is essential, whether or not incumbents or past employees actually completed the specific function, the time spent performing the function, the consequences of not performing the function, and in unionized environments, the terms of collective bargaining agreements.
Some job duties do not meet the essential standard. These are called marginal job functions. Marginal job functions are duties that are not central to the position. A qualified individual with a disability does not have to be able to perform marginal functions of a position in order to be qualified. While an employer is under no obligation to remove essential job functions as an accommodation, it is reasonable to ask that marginal job functions be reassigned.