The letter assessing a person's disability for a medical committee of the National Insurance Institute, is the opinion – given as part of private practice - of a physician who is an expert in his field in regard to the impairment causing the disability. The disability assessment letter for the purpose of claiming disability benefits from the National Insurance Institute should be written by a physician who is an expert in the field of the impairment for which the disability benefits are claimed. A disability assessment letter is based on medical records related to the impairment, tests and additional imaging results which may be requested by the expert during the physical examination for the purpose of issuing a disability assessment and classification. The expert physician's experience allows him to further assess the duration of the disability, the treatment it might require and the future changes, if any, predicted for this impairment.

A medical committee of the National Insurance Institute is composed of a physician employed by the National Insurance Institute, with expertise appropriate to the impairment for which disability benefits are claimed, and a secretary whose role is to maintain a protocol and protect the claimant's rights.

"Second Opinion", is as its name says, another opinion in addition to that of the physician appointed by the National Insurance Institute who serves at the medical committee, who decides on the disability benefits. The National Insurance Institute's physician is not required to accept the opinion of the private practitioner who gave the "Second Opinion" (the disability assessment letter), but he is required to take it into consideration and provide an explanation as to why his opinion differs from it, and therefore it is important to correctly select the expert for providing such as opinion.