Calculate the average number of the word occurrences in which ‘system’ appeared as the first word in the pair.
average word occurrences = divide(sum(occurrences), count(occurrences)); ‘system’ appeared as the first word refers to w1st = 'system'
SELECT AVG(T2.occurrences) FROM words AS T1 INNER JOIN biwords AS T2 ON T1.wid = T2.w1st WHERE T2.w1st = ( SELECT wid FROM words WHERE word = 'sistema' )