Further , to allow for greater context richness , the second analysis was based on the titles and abstracts of selected articles ( VanEck and Waltman , 2010 ) . Similar to the procedure based on keywords and with the same criteria , the minimum number of occurrences of words was set to three . The data was also cleaned by elimination or grouping ( VanEck and Waltman , 2010 ) . For example , generic or irrelevant words , such as " article , " " item , " " author , " and " study , " were eliminated . However , similar terms were grouped together , as in the case of " covid , " " covid-19 , " and " pandemic . " Fig . 6 shows the results of the co - occurrence analysis based on titles and abstracts .