The compound concentration or peak intensity values are assumed to be non-negative values. However, metabolomics data may contain some negative values due to various reasons (such as baseline subtraction). In order to deal with these values, MetaboAnalyst adopts a simple variation used in generalized logarithm (glog). It has many desirable features (for details, see Durbin BP. et al). Its formula is shown below:
In MetaboAnalyst, the log base is 10, and a is one tenth of the minimal positive values in the data.