What is interval data?

Interval data is continuous data where differences are interpretable, but where there is no "natural" zero. A good example is temperature in Fahrenheit degrees.

Ratios are meaningless for interval data. You cannot say, for example, that one day is twice as hot as another day.

Example: In a study of a seven item pictorial scale for three aspects of dyspnea (throat closing, chest tightness, and effort), children were asked to place these images on along a visual analog scale.

Results Children aged eight years or older rated the scales in the correct order 75% to 98% correctly, but children less than 8 years of age performed unreliably. The mean distance between each consecutive item in each pictorial scale was equal. Conclusion Preliminary results revealed that children aged 8 to 18 years understood and used these three scales measuring throat closing, chest tightness, and effort appropriately. The scales appear to accurately measure the construct of breathlessness, at least at an interval level. Additional research applying these scales to clinical situations is warranted. -- Dalhousie dyspnea scales: construct and content validity of pictorial scales for measuring dyspnea. Patrick J McGrath, Paul T Pianosi, Anita M Unruh and Chloe P Buckley. BMC Pediatrics 2005, 5:33doi:10.1186/1471-2431-5-33. [Medline] [Abstract] [Full text] [PDF]

This work is licensed under a Creative Commons Attribution 3.0 United States License. It was written by Steve Simon on 2002-10-11, edited by Steve Simon, and was last modified on 2008-01-12. Send feedback to ssimon at cmh dot edu or click on the email link at the top of the page. Category: Definitions.