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Dear Allstatters, I am analysing a data set which is continuous on a closed interval [a,b]. Globally, this data is symmetrically distributed across the interval, but I need to prepare summaries at a local level (such as postcodes) including an estimate with a confidence interval. Naturally to avoid silly confidence intervals which breach the interval [a,b], I need to transform the interval to an open unbounded one instead. If the interval was open i.e. (a,b) then the logit transformation Log( (x-a)/b-x) ) would be perfect but clearly this is not suitable when x equals a or b. I have been unable to find a suitable alternative so I am proposing to use the transformation Log( (x-a+k)/(b-x+k) ) where k is a small number say 1% of b-a. I have used this in the past and I recognise that this is not perfect solution so I would be grateful if someone from the ALLSTAT list could suggest a better transformation. The best I have found so far is this article http://biostatistics.oxfordjournals.org/cgi/reprint/8/1/72 but I don't think this is quite what I am looking as it doesn't deal with continuous data on [a,b]. Regards Nigel Marriott Chartered Statistician <http://www.marriott-stats.com/> www.marriott-stats.com Ground Floor, 21 Marlborough Buildings, Bath BA1 2LY, United Kingdom Tel (mobile) +44 (0)773 4069997 Tel (office) +44 (0)1225 489033 Fax +44 (0)870 6221969 Marriott Statistical Consulting Limited, Company No. 5577275, VAT No. 883304029 Registered in England, Registered Office - Equity House, 4-6 School Road, Tilehurst, Reading, RG31 5AL