The NIST 2017 library was used as the training set. Preprocessing of the library is described in our previous work , and unsupported compounds were excluded as described there. For each compound, the median value of all values for PEG was used. The compounds that were also measured In the SEQ_ADD method, at the first stage, the most correlated with the target RI values MD is selected. Then for each of MD that have not yet been selected, the ordinary least squares (OLS) model is built and the MD for which the f-factor (goodness of fit) is the largest is selected. In this method, the f-factor was calculated using the following equation: