The absolute difference in the frequency of harmful outcomes between experimental and control groups, calculated as frequency of harmful outcomes in the experimental group minus the frequency of harmful outcomes in the control group. Typically used to describe a harmful exposure or intervention (e.g., if the frequency of adverse outcomes is 20% in treatment and 10% in control, the absolute risk increase would be 10% expressed as a percentage and 0.10 expressed as a proportion).
Event rates have to do with the frequency of occurrence of an event in a population. Event rates take into account the possibility of an event occurring several times in an individual and are typically expressed as events per 100 person years or 1,000 person years. The term “event rate” is very frequently, but incorrectly, used to describe the frequency or risk of events when an event is counted only once in an individual. Most of the time when you see “control event rate” what is meant is “control group risk”; similarly when you see “experimental event rate” what is usual meant is the risk in the experimental group.