Mixed effects model Repeated measures Spline with one knot BUT location of the knot is different for everyone (like a random intercept of the knot) AND a variable in the data set predicts location of the knot. Can it be done (STATA or lmer)? #stats

i can't vouch for anything here but this might help cran.rstudio.com/web/packages...

segmented: Regression Models with Break-Points / Change-Points Estimation (with Possibly Random Effects)

Fitting regression models where, in addition to possible linear terms, one or more covariates have segmented (i.e., broken-line or piece-wise linear) or stepmented (i.e. piece-wise constant) effects. ...

What do you mean by a spline with one knot? All the splines I know of require at minimum two or three knots to define

tvem package maybe?