The assumption underlying the LMS method is that after Box-Cox power transformation the data at each age are normally distributed.
Clearly a cut off point related to age is needed to define child obesity, based on the same principle at different ages, for example, using reference centiles.10 In the United States, the 85th and 95th centiles of body mass index for age and sex based on nationally representative survey data have been recommended as cut off points to identify overweight and obesity.11 For wider international use this definition raises two questions: why base it on data from the United States, and why use the 85th or 95th centile?
Other countries are unlikely to base a cut off point solely on American data, and the 85th or 95th centile is intrinsically no more valid than the 90th, 91st, 97th, or 98th centile.
The hypothesis is that the relation between cut off point and prevalence at different ages gives the same curve shape irrespective of country or obesity.
If sufficiently similar the curves can be averaged to provide a single smooth curve passing through the adult cut off point.
The resulting curves were averaged to provide age and sex specific cut off points from 2-18 years.