What TRIR, DART and LTIFR mean
TRIR usually refers to the total recordable incident rate, calculated on a base of 200,000 hours in the OSHA context. DART covers cases involving days away from work, restricted work or transfer to another job, following the relevant categories.
LTIFR is used for lost-time injury frequency, often on a base of one million hours. The specific definitions of injury, lost time and scope must be stated. The variants LTIR and LTIFR should not be interpreted from their acronyms alone: always check the method declared by the organisation or reference standard.
Formula and calculation base
The general structure is rate = number of cases × base / hours worked. In the explanation provided by the US Bureau of Labor Statistics, the 200,000-hour base represents 100 people working 40 hours per week for 50 weeks.
DART counts cases, not accumulated days. The numerator and hours must relate to the same period and population. Actual hours worked exclude holidays, sick leave and other time not worked, even when paid. If some hours are estimated, document the basis so the calculation can be reproduced.
What constitutes a recordable case
In the OSHA system, recordability is determined by specific rules. General criteria include, among others, death, days away from work, restricted work or job transfer, medical treatment beyond first aid and loss of consciousness. Additional criteria and specific situations also apply.
An “OSHA recordable case” should not automatically be equated with an “accident involving sick leave” in another national system. An international rate required by a client may need an additional classification without replacing locally required records or notifications. Keep the classification criteria and evidence, and avoid changing definitions to improve the result artificially.
How to compare sites and countries
Before comparing, use a common methodology sheet: case definition, base, period, people included, source of hours and treatment of contractors. If some figures include contractors and others do not, the comparison may be misleading.
The same numerator produces a rate five times higher with a one-million-hour base than with a 200,000-hour base. That conversion only addresses scale; it does not correct differences in definitions. Size, activity and exposure also matter. In small populations, a single event can substantially change the rate. Show the cases and hours alongside the indicator so it can be interpreted.
Limitations and common errors
A low rate does not, by itself, demonstrate that risks are controlled. It may coexist with high-potential incidents, undetected exposures or reporting problems. Complement it with indicators of preventive controls and analysis of event severity and mechanisms.
Common errors include adding days instead of cases, mixing injuries and illnesses without explanation, using employee hours for events that include contractors, or comparing individual months with annual cumulative figures. Establish reviews when a case classification is updated, and retain a traceable record of corrections without penalising complete reporting.
Hypothetical calculation example
A site records six cases meeting its TRIR criteria and 600,000 hours worked. The rate is 6 × 200,000 / 600,000 = 2.0. Two of those cases meet the DART criteria: 2 × 200,000 / 600,000 = 0.67, rounded to two decimal places.
If one case meets the documented definition of a lost-time injury and a one-million-hour base is adopted, LTIFR would be 1 × 1,000,000 / 600,000 = 1.67. These are not three contradictory readings: they describe different sets and bases. The report must accompany them with their definitions, period and included population.
