Design and methodological considerations in the National Cooperative Gallstone Study: A multicenter clinical trial

John M. Lachin, Jay W. Marks, Leslie J. Schoenfield, Protocol Committee NCGS Protocol Committee, Malcolm P. Tyor, Peter H. Bennett, Scott M Grundy, William G M Hardison, Lawrence W. Shaw, Johnson L. Thistle, Z. R. Vlahcevic, Cooperative Gallstone Study Group National Cooperative Gallstone Study Group