Hello fellow postgrads,

I have been searching for the answer for a problem with confidence intervals for a long time now, so I hope someone can help!

The two papers below performed the same analysis, regressing irregular word reading onto reading nonword reading for controls, and plotting these regressions with 90% confidence intervals, before superimposing dyslexic participants to find outliers. However, one paper has straight line intervals, the other curved intervals. Why?

I have scoured both textbooks and online resources, and the only descriptions of confidence intervals I have found are for confidence intervals of the regression line, which estimate an area in which there is a given probability that the regression line will fall. This is not what the studies below used, however. They used what may, I think, be termed ‘prediction intervals’ and describe an area in which a given percentage of the data will fall. Can anyone help with these distinctions, and/or provide a calculation for this type of interval? The curious thing is that SPSS/PASW will plot these confidence intervals for me, but offers no explanation of where they come from, nor can I edit the provided plot.

Any help, no matter how small, will be greatly appreciated!

References:

Manis, F. R., Seidenberg, M. S., Doi, L. M., McBride-Chang, C. & Petersen, A. (1996). On the bases of two subtypes of developmental dyslexia. Cognition: 58, 157-195.

Stanovich, K.E., Siegel, L.S. & Gottardo, A. (1997). Converging evidence for phonological and surface subtypes of dyslexia. Journal of Educational Psychology: 89, 114-127.

Regards,

Mr. Dean Wybrow

Dean Wybrow BSc, MRes
Department of Psychology
University of Essex
Colchester CO4 3SQ

Tel: +44 (0)1206 - 872575

www.essex.ac.uk/psychology/department/people/wybrow.html

essex.academia.edu/DeanWybrow/About

'In all science, error precedes the truth, and it is better it should go first than last.'  -Hugh Walpole