Key Definitions of Statistical Terms
Prepared September 2008
- Vision loss refers to individuals who have trouble seeing, even when wearing glasses or contact lenses, as well as to individuals who are blind or unable to see at all.
- Clinically diagnosed vision loss is determined by an optometrist or ophthalmologist using dilated eye exams and standard measurement tools.
- Self-reported vision loss is determined on an individual basis based on that person's perceived visual ability and its effect on daily functioning.
- Legal blindness is a level of vision loss that has been legally defined to determine eligibility for benefits. The clinical diagnosis refers to a central visual acuity of 20/200 or less in the better eye with the best possible correction, and/or a visual field of 20 degrees or less. Often, people who are diagnosed with legal blindness still have some useable vision.
- Total blindness refers to an inability to see anything with either eye.
- Low vision is a term often used interchangeably with visual impairment and refers to a loss of vision that may be severe enough to hinder an individual's ability to complete daily activities such as reading, cooking, or walking outside safely, while still retaining some degree of useable vision.
- Visual impairment is often defined clinically as a visual acuity of 20/70 or worse in the better eye with best correction, or a total field loss of 140 degrees. Additional factors influencing visual impairment might be contrast sensitivity, light sensitivity, glare sensitivity, and light/dark adaptation.
- Functional limitation refers to the interaction of visual functioning and ability to perform activities of daily living/instrumental activities of daily living. Common daily activities affected by vision loss are reading, safe pedestrian travel, self-care, cooking, and recreational activities.
- Visual acuity is the clinical measure of the eye's ability to distinguish details of the smallest identifiable letter or symbol. This measurement is usually given in a fraction and is based upon visible print size. Typical vision is 20/20. If an individual sees 20/200, the smallest letter that this individual can see at 20 feet could be seen by someone with typical vision at 200 feet./li>
- Frequency is the number of people who were similar for a given characteristic. For example, 25.2 million adult Americans reported vision loss.
- Prevalence is a measurement of all of the individuals in a population affected by a condition within a particular period of time. Prevalence indicates how widespread a condition is. Prevalence also means "proportion" and is typically expressed as a percentage. For example, 21.7% of Americans 75 years of age and over reported having vision loss.
- Incidence is a measurement of the number of new individuals who develop a condition during a particular period of time. Incidence conveys information about the risk of developing the condition. For example, approximately 200,000 people develop neovascular age-related macular degeneration each year in the U.S.