FI was developed by Dr. Kenneth Rockwood and Dr. Arnold Mitnitski at Dalhousie University in Canada.

FI is defined as the proportion of deficits present in an individual out of the total number of age-related health variables considered.

These deficits include diseases, signs and symptoms, laboratory abnormalities, cognitive impairments, and disabilities in activities of daily living.

The FI takes advantage of the high redundancy in the human organism.

The standard procedure for creating a frailty index are found in an open-access publication.