Is a 95% CI on a breakpoint fixed effect legitimate when the nonlinear equation is continuous but not differentiable at the breakpoint? I used the nlme to generate a non-linear mixed-effects piecewise model with initial slope zero prior to an unknown breakpoint. It did give a 95% CI for the breakpoint. Thanks Erik whole model is complex, but here is the simple equation used: dr.hockey<-function(a,b,x,brk){ a+ifelse(x<=brk,0,b*(x-brk))}
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