The document discusses the reliability of educational assessments and how the public has a weak understanding of reliability. It argues that all assessments inherently have some level of unreliability due to human factors and limitations in measurements. The document advocates that those producing and mandating assessments must take responsibility for ensuring the informed use of results and promoting understanding of reliability issues and limitations.

2. The reliability of educational assessments Dylan Wiliam www.dylanwiliam.net Ofqual Annual Lecture, Coventry: 7 May 2009

10. Test length and reliability From To Just about the only way to increase the reliability of a test is to make it longer, or narrower (which amounts to the same thing). 0.70 0.75 0.80 0.85 0.90 0.95 0.70 1.0 0.75 1.3 1.0 0.80 1.7 1.3 1.0 0.85 2.4 1.9 1.4 1.0 0.90 3.9 3.0 2.3 1.6 1.0 0.95 8.1 6.3 4.8 3.4 2.1 1.0