Medical researchers are concerned about the association between maternal smoking habits and the need for caesarian section during birth. Suppose the researchers gather 423 expecting mothers in their third trimester and follow them through pregnancy to examine the incidence of caesarian. Suppose out of the 102 mothers that reported smoking cigarettes at least once during pregnancy, 59 of them were referenced to have needed caesarian intervention during delivery, while 73 of the mothers with no smoking history reportedly needed the procedure. What is the risk difference for caesarian among smokers during pregnancy versus non-smokers?