What's so important about recordkeeping. CALCULATION OF PERSON-TIME AND INCIDENCE RATES. Example 1 Observe 1st graders, total 500 hours. Observe 12 accidents. Accident rate (or Accident. Jun 2, 2009. Calculation of Incident Rate, Injury Severity Rate & Injury Frequency Rate. Hi all, I guess it's inevitable that one day, accident might happen in.