When 90% confidence intervals are 50% certain: on the credibility of credible intervals
Article first published online: 14 MAR 2005
Copyright © 2005 John Wiley & Sons, Ltd.
Applied Cognitive Psychology
Volume 19, Issue 4, pages 455–475, May 2005
How to Cite
Teigen, K. H. and JØrgensen, M. (2005), When 90% confidence intervals are 50% certain: on the credibility of credible intervals. Appl. Cognit. Psychol., 19: 455–475. doi: 10.1002/acp.1085
- Issue published online: 21 APR 2005
- Article first published online: 14 MAR 2005
- Research Council of Norway. Grant Number: 135854/350
Estimated confidence intervals for general knowledge items are usually too narrow. We report five experiments showing that people have much less confidence in these intervals than dictated by the assigned level of confidence. For instance, 90% intervals can be associated with an estimated confidence of 50% or less (and still lower hit rates). Moreover, interval width appears to remain stable over a wide range of instructions (high and low numeric and verbal confidence levels). This leads to a high degree of overconfidence for 90% intervals, but less for 50% intervals or for free choice intervals (without an assigned degree of confidence). To increase interval width one may have to ask exclusion rather than inclusion questions, for instance by soliciting ‘improbable’ upper and lower values (Experiment 4), or by asking separate ‘more than’ and ‘less than’ questions (Experiment 5). We conclude that interval width and degree of confidence have different determinants, and cannot be regarded as equivalent ways of expressing uncertainty. Copyright © 2005 John Wiley & Sons, Ltd.