Dear PaigeMiller, Thank you for your reply. My goal is to apply a 2 piecewise linear regression model (left graph on the joint picture) to the Data presented in the previous mail. In order to estimate the Timevar (x value) corresponding to the changepoint, I wanted to adopt the same approach as the one used in the article referred below. Approach : estimate a changepoint by minimizing the residuals sum of squares (cf right graph in the picture). If the residuals sum of square is a single number and does not vary depending on x values, I misinterpreted the figure 1. Do you think they have calculated the residuals sum of squares values for models with different changepoints and selected the one with the lowest residuals sum of squares value? If so, do you have an idea how to reproduce it with SAS? Thank you again for your help and guidance. HaddadMez article reference : Greene, M.E., Rolfson, O., Garellick, G., Gordon, M., Nemes, S., 2015. Improved statistical analysis of pre- and post-treatment patient-reported outcome measures (PROMs): the applicability of piecewise linear regression splines. Qual Life Res 24, 567–573. https://doi.org/10.1007/s11136-014-0808-3
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