I'm working on a meta-analysis regarding the effect of interventions on the incidence of aggression against health workers. One study in particular compares the percentage of staff of that reports having been the victim of incidents of aggression in the month prior to intervention and in the month following it in two wards: the intervention ward and the control ward. So, 22% of the staff in the intervention group reports having been the victim of aggression in the prior month, compared to 19% post-intervention. For the control group, these numbers are 13% and 8% respectively. I'm thinking that the most straightforward way to do it is comparison of relative change in risk between the control and intervention groups, but am having a hard time finding information on this.