THE CAMBRIDGE STATISTICS DISCUSSION GROUP


                Thursday 5th October 2000 7:15 for 7:45



                       Hopkinson Room,
         University of Cambridge Computer Laboratory,                         
                 Pembroke Street, Cambridge




         Sympercents: A Simplified Presentation of Natural 
                     Log Transformed Data         
        





                         Tim Cole,
             Institute of Child Health, London
                    


Abstract: The results of analyses on log transformed data are usually
back-transformed and interpreted on the original scale. Yet if natural logs
are used this is not necessary - the log scale can be interpreted as it
stands. A difference of natural logs corresponds to a fractional difference
on the original scale. The agreement is exact if the fractional difference
is based on the logarithmic mean. The transform y = 100 loge x leads to
differences, standard deviations and regression coefficients of y that are
equivalent to symmetric percentage differences, standard deviations and
regression coefficients of x. Several simple clinical examples show that
the 100 loge scale is the natural scale on which to express percentage
differences. The term sympercent or s% is proposed for them. Sympercents
should improve the presentation of log transformed data and lead to a wider
understanding of the natural log transformation.






Speaker: 
Tim started out as an engineer, taking the Mechanical Sciences tripos at
Cambridge. He then spent two years in Oxford and became a Bachelor of
Philosophy in Statistics (a rare qualification - there are only 5 extant as
the BPhil soon changed to MPhil).

In 1970 he joined the MRC Pneumoconiosis Unit in South Wales, where he
spent 5 happy years learning at the feet of Peter Oldham. He then moved to
the MRC Dunn Nutrition Unit in Cambridge, remaining there until its major
reorganisation in 1998. Still MRC funded he moved to the Institute of Child
Health in London, where he now has a personal chair in medical statistics.


Annual General Meeting: Tim's talk will be preceded by a brief
Annual General Meeting.

Directions: Enter the New Museums site through the arch from Pembroke Street.
Turn to the left in front of the building with the brown iron staircase
and pass through the passage way by the tree. Enter the lobby which is
straight in front of you and find the back entrance to the Computer Service
on the left. When inside follow the signs. It is not possible to park on
the New Museums site in the evening. The back entrance to the Computer 
Service will be locked at 7:45.


Next Meetings:
30th November - David Clayton (Biostatistics Unit) on 'Approaches to the 
analysis of longitudinal studies with dropout: cognitive function and aging'.
8th February - Graham Healey (Huntingdon Life Sciences).

Supper: Some members eat regularly in the University Centre before
each meeting at 6-15. Feel free to join them. 

Secretary: Peter Watson, MRC Cognition and Brain Sciences Unit, 
15 Chaucer Road, Cambridge CB2 2EF;telephone 01223 355294 Extension 380; 
E-mail [log in to unmask]







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