# How to choose cut off for winsorization/ flooring- capping? What is the impact of variable distribution on the decision

To perform logistic regression I wish to winsorize outliers in independent/ explanatory variables by flooring and capping independent variables. Can you suggest how I should choose cut-off for winsorization, i.e 1 & 99th or something else. Also is there any impact on decision if the distribution of variable is left/ right skewed and not normal.

Please assume we don't have common knowledge of the variable to choose cut off intuitively.

• This seems to be an unusual (and potentially biased) approach to logistic regression. Would you have any references recommending and justifying its use? – whuber Apr 8 '19 at 22:25
• (1) Automatically throwing out outliers based on a percentile and (2) winsorizing explanatory variables in regressions are both unusual procedures and require careful justification. – whuber Apr 8 '19 at 22:57
• What problem are you trying to solve with the capping? That should help guide us towards a principled solution. – Matthew Drury Apr 8 '19 at 23:37
• @whuber: The help page for glmrob claims it can do this, see the argument weights.on.x (with default value none). It can even take a function as arguments, so can be used as a basis for own implementation. But I didn' test it ... here is a promising paper. – kjetil b halvorsen Apr 10 '19 at 21:26
• @kjetilbhalvorsen Thank you! I hadn't noticed that option, and it's precisely what is needed. – whuber Apr 11 '19 at 13:06