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the Percentile is non-fixed length that is affected by the count and compression mainly. so we collect some statistics
about it and do some analysis, which get from test and the T-digest Paper
As a result, we conclude its regular pattern by Stata , a tool help construct function model.
0 - 2 * compression it grows a linear function which is easily derived from T-digest Algorithm
2 * compression - 50000000 it grows roughly a log and the accuracy increases with the number of samples observed