study source       n1i n2i ai ci
1 "Chopra et al."   39  43  2  1
2 "Mogensen"        44  44  4  4
3 "Pitt et al."    107 110  6  4
4 "Darby et al."   103 100  7  5
5 "Bennett et al." 110 106  7  3
6 "O'Brien et al." 154 146 11  4
