Comment on the CUSUM surgical learning curve analysis in Dimitrovska et al. (2022).

The cumulative sum (CUSUM) analysis included in the recent paper by Dimitrovska et al. [1] illustrates the major pitfall of using this popular approach to reach conclusions about the surgical learning process. Below we refer to the time series plot of operation times (Figure 1 [1]) and the corresponding CUSUM plot (Figure 2 [1]) from their paper [1]. The CUSUM plot of the sum of the successive differences of the operating times from their average (Figure 2 [1]) was broken into 3 phases, i.e. the initial learning, the increased competence and the experienced phases. As pointed out by Woodall et al. [2], however, this type of plot is subject to over- interpretation. It has neither a conceptual nor a mathematical justification.