Figure 1.

Bubble map showing words or phrases that appeared in at least ten of the 280 papers. We excluded generic terms by a visual inspection of the map generated in the preliminary stage. After screening, 123 terms remained. The bubble size indicates the number of papers containing the term. The bubble color indicates its citation per paper. If the terms frequently co-appeared in the same papers, their bubbles were in closer proximity. The x- and y-axis carried no specific information and only proximity between bubbles should be considered. Some of the measurement indices might appear under different names.