How to refer to descriptive information of variables as exogenous variables
This may depend on the field, but to me the phrase "the descriptive information of main variables and item pool characteristics" is utterly unclear. If you don't want to spell out the full set of descriptive statistical operators you use, consider including something like: "Our exogenous variables encompass [...] summary statistics over the main variables". 

However, in the interest of reproducibility, I would personally prefer to spell out the full set. Right now, it is unclear to me how far you stretch up the set {min, max, mean, std}. There are so many things that could be included here: median, interquartile range, skewness, kurtosis, higher-order moments, etcetera. It's best to be explicit about your setup.