For each synthesis protocol, a vector was generated using the term frequency-inverse document frequency algorithm (TF-IDF), a commonly used text mining method to estimate the importance of words in a group of documents. The TF-IDF algorithm weights the frequency of a word used in each document against its frequency across the group of documents -words present in many documents are given a low weight, while words occurring in only rarely are given a high weight. This is shown in Equation , which calculates the weight of word t in the individual document d as part of the group of documents D, where f is the frequency the word occurs. As in this study the "words" are chemical names, common chemicals like methanol are afforded a low weight, while rarer chemicals like CTAB are afforded a relatively higher weight.