Incidence, Frequency and Severity Rates
5 minutes
5 Questions
In Certified Construction Manager (CCM) practice, safety performance is measured with three core lagging indicators: Incidence Rate, Frequency Rate, and Severity Rate. Each normalizes injury data to hours worked so that projects, contractors, and companies of different sizes can be compared fairly. Incidence Rate (often the OSHA Recordable Incidence Rate, or TRIR) counts how many OSHA-recordable injuries and illnesses occur per 200,000 employee-hours. The figure 200,000 represents 100 full-time workers laboring 40 hours per week for 50 weeks. The formula is: Incidence Rate = (Number of recordable cases × 200,000) ÷ Total hours worked. A rate of 3.0, for example, means three recordable cases for every 100 full-time workers per year. Owners frequently use this rate, along with the related DART rate (Days Away, Restricted, or Transferred), during contractor prequalification. Frequency Rate focuses on how often lost-time injuries occur. Historically based on the ANSI Z16.1 standard, it uses 1,000,000 hours as the base: Frequency Rate = (Number of lost-time injuries × 1,000,000) ÷ Total hours worked. It answers the question, 'How frequently are workers hurt badly enough to miss work?' Severity Rate measures how serious those injuries are by counting days lost rather than cases: Severity Rate = (Total days lost or charged × 1,000,000) ÷ Total hours worked. Two contractors may have identical frequency rates, but the one whose injuries result in weeks of lost time has a much higher severity rate, signaling more serious hazards. For the construction manager, these rates serve several purposes: benchmarking against Bureau of Labor Statistics industry averages, evaluating subcontractor safety culture, tracking trends across a project's duration, influencing Experience Modification Rates (EMR) and insurance premiums, and identifying where corrective actions or additional training are needed. Because they are lagging indicators, effective CCMs pair them with leading indicators such as safety audits, toolbox talks, and near-miss reporting to proactively prevent incidents rather than merely record them.
Incidence, Frequency and Severity Rates — CMAA CCM Safety Management Study Guide
1. Why This Topic Matters
Safety Management is one of the core domains of the CMAA Certified Construction Manager (CCM) exam, and the construction manager is expected to measure safety performance, not just talk about it. Incidence, frequency and severity rates are the standardized statistics that allow a CM to:
• Prequalify and compare contractors of very different sizes on an equal footing
• Benchmark a project or a company against industry averages published by the U.S. Bureau of Labor Statistics (BLS) and OSHA
• Track trends over the life of a project and trigger corrective action when the numbers move in the wrong direction
• Report to owners, insurers and regulators in a language they recognize
• Understand how a contractor's safety history drives its insurance cost through the Experience Modification Rate (EMR)
Because these are formula-driven, they are among the most predictable calculation questions on the exam. A candidate who memorizes two constants and understands what each rate measures can reliably collect these points.
2. What the Rates Are
All three rates share the same idea: take a count of something bad (cases or lost days), divide by the exposure (hours worked), and multiply by a constant so the answer is expressed per a standard-sized workforce. This normalization is what lets a 20-person subcontractor be compared with a 5,000-person general contractor.
A. OSHA Incidence Rate (also called Total Recordable Incident Rate, TRIR, or Recordable Incidence Rate)
Incidence Rate = (Number of OSHA recordable injuries and illnesses × 200,000) ÷ Total employee hours worked
• The constant 200,000 represents 100 full-time employees working 40 hours per week for 50 weeks (100 × 40 × 50 = 200,000). The result is therefore "recordable cases per 100 full-time workers per year."
• Recordable cases come from the OSHA 300 Log and include: fatalities, days away from work, restricted work or job transfer, medical treatment beyond first aid, loss of consciousness, and other significant injuries or illnesses diagnosed by a licensed health care professional.
• Variants using the same 200,000 base: DART rate (Days Away, Restricted or Transferred cases) and LTIR / Lost Time Incident Rate (cases with days away from work only).
• For reference, the construction industry TRIR published by BLS has generally been in the range of roughly 2.5 to 3.0 in recent years; a contractor well below that figure is performing better than average.
B. Frequency Rate (traditional ANSI Z16.1 method)
Frequency Rate = (Number of lost-time (disabling) injuries × 1,000,000) ÷ Total employee hours worked
• The constant 1,000,000 expresses the result as disabling injuries per million hours worked.
• Only lost-time injuries (not every recordable) are counted. This older method is still widely used by contractors, insurers and in CMAA study material, so you must know it alongside the OSHA method.
C. Severity Rate
Severity Rate = (Total days lost or charged × 1,000,000) ÷ Total employee hours worked
• Measures how serious the injuries were, not how many. One fatality or one permanent disability can produce a very high severity rate even when the frequency rate is low.
• "Days charged" includes scheduled charges for permanent injuries (for example, ANSI Z16.1 assigns 6,000 days to a fatality or permanent total disability).
• Some organizations compute severity on the OSHA 200,000-hour base (days lost per 100 workers). If an exam question states the base, use the base the question gives you.
D. Average Days per Lost-Time Case
Average Severity = Total days lost ÷ Number of lost-time cases = Severity Rate ÷ Frequency Rate
This is a convenient check and a frequent follow-up question.
E. Experience Modification Rate (EMR)
Not a rate you calculate on the exam, but closely related. The EMR is set by the insurance rating bureau (NCCI or a state bureau) from three years of workers' compensation loss history, excluding the most recent year. An EMR of 1.0 is industry average; below 1.0 means better-than-average experience and lower premiums; above 1.0 means worse-than-average and higher premiums. Owners and CMs commonly set a maximum EMR (often 1.0) as a prequalification threshold.
3. How It Works in Practice
Step 1 – Gather exposure data. Total hours worked by all employees for the period, from payroll or certified payroll reports. Do not include vacation, sick leave or holiday hours; include overtime actually worked.
Step 2 – Gather case data. From the OSHA 300 Log and 300A annual summary: number of recordables, number of DART cases, number of lost-time cases, and total calendar days away or restricted.
Step 3 – Apply the formula. Multiply the count by the constant first, then divide by hours. This avoids rounding errors.
Step 4 – Interpret. Compare against the contractor's prior periods, the project target, the BLS/NAICS industry average, and the owner's prequalification criteria. Rising incidence with flat severity suggests many minor injuries (housekeeping, PPE, training issues); low incidence with a severity spike suggests a single serious event that warrants a root-cause investigation.
Step 5 – Act. The CM's role is to use the data in the project Safety Management Plan: adjust toolbox-talk topics, increase inspections, require corrective action plans, or in extreme cases recommend removal of a subcontractor. Remember that these are lagging indicators; a mature program pairs them with leading indicators such as near-miss reports, inspection scores, training hours and job hazard analyses completed.
4. Worked Examples
Example 1 – Incidence Rate. A contractor records 6 OSHA recordable injuries while its employees work 480,000 hours.
TRIR = (6 × 200,000) ÷ 480,000 = 1,200,000 ÷ 480,000 = 2.5 recordables per 100 full-time workers.
Example 2 – Converting headcount to hours. A project averages 50 workers for one year. Assume 2,000 hours per worker per year (40 hrs × 50 wks).
Hours = 50 × 2,000 = 100,000. If there were 2 recordables: TRIR = (2 × 200,000) ÷ 100,000 = 4.0.
Example 3 – Frequency and Severity. Over 500,000 hours, a subcontractor has 3 lost-time injuries totaling 90 days lost.
Frequency Rate = (3 × 1,000,000) ÷ 500,000 = 6.0 lost-time injuries per million hours.
Severity Rate = (90 × 1,000,000) ÷ 500,000 = 180 days lost per million hours.
Average days per case = 90 ÷ 3 = 180 ÷ 6 = 30 days.
Example 4 – Working backwards. A contractor reports a TRIR of 1.5 on 800,000 hours. How many recordables?
Cases = (TRIR × Hours) ÷ 200,000 = (1.5 × 800,000) ÷ 200,000 = 6 recordables.
Example 5 – Comparing contractors. Contractor A: 4 recordables in 200,000 hours (TRIR 4.0). Contractor B: 12 recordables in 1,200,000 hours (TRIR 2.0). Although B had three times as many injuries, B is the safer performer because its rate is half that of A. This is exactly why rates, not raw counts, are used for prequalification.
5. Exam Tips: Answering Questions on Incidence, Frequency and Severity Rates
Tip 1 – Memorize the two constants and what they mean. 200,000 = OSHA incidence/TRIR/DART (per 100 workers). 1,000,000 = traditional frequency and severity (per million hours). If the question says "OSHA" or "recordable," think 200,000. If it says "frequency rate," "severity rate" or "per million hours," think 1,000,000. If the question explicitly gives a base, use the one given even if it differs from what you memorized.
Tip 2 – Identify the numerator carefully. Incidence and frequency rates use a count of cases. Severity uses days lost or charged. Read whether the question asks about all recordables, lost-time cases only, or DART cases; the answer choices will often include a distractor computed with the wrong numerator.
Tip 3 – Make sure exposure is in hours. If the question gives workers, weeks or months, convert to hours first. Standard conversions: 2,000 hours per worker-year, 40 hours per worker-week. Watch for questions that give a project duration in months and a crew size; multiply crew × hours/week × weeks.
Tip 4 – Multiply before you divide. Computing cases × 200,000 first and then dividing by hours keeps the arithmetic clean and reduces rounding errors that can push you toward a wrong answer choice.
Tip 5 – Sanity-check the magnitude. A construction TRIR is usually a single-digit number (roughly 1 to 6). A frequency rate per million hours is often 5 to 25. Severity rates run into the hundreds or thousands. If your TRIR comes out as 250, you probably used 1,000,000 instead of 200,000, or forgot to convert workers to hours.
Tip 6 – Know the interpretation questions, not just the math. Expect items such as: "Which rate best measures how serious injuries were?" (severity); "Why are rates used instead of raw injury counts?" (to normalize for workforce size and allow comparison); "Which document is the source of recordable case data?" (OSHA 300 Log / 300A summary); "Are these leading or lagging indicators?" (lagging).
Tip 7 – Understand EMR direction. Lower is better. 1.0 is average. An EMR of 0.80 means premiums roughly 20 percent below the base; 1.25 means 25 percent above. EMR is calculated by the insurance rating bureau, not by the contractor, and it reflects a three-year window that excludes the most recent policy year.
Tip 8 – Know the CM's role. On a CMAA exam the "best" answer usually positions the CM as the party who monitors, reviews and reports safety performance and enforces the owner's prequalification criteria, while the contractor retains primary responsibility for means, methods and site safety. Do not choose answers that have the CM directing the contractor's safety means and methods.
Tip 9 – Watch for the average-days-per-case trick. Severity Rate ÷ Frequency Rate = average days lost per lost-time case. If a question gives you both rates and asks for average days per injury, do not recompute from scratch; just divide.
Tip 10 – Recognize limitations when asked. Rates can be distorted by underreporting, small hour totals (one injury on a tiny project yields a huge rate), differences in recordkeeping criteria, and the time lag between a hazard and a recorded injury. The textbook answer is to use them together with leading indicators and to evaluate trends over multiple periods rather than a single number.
6. Quick Reference Card
• TRIR / Incidence Rate = (Recordables × 200,000) ÷ Hours
• DART Rate = (Days Away/Restricted/Transfer cases × 200,000) ÷ Hours
• LTIR = (Lost-time cases × 200,000) ÷ Hours
• Frequency Rate = (Lost-time injuries × 1,000,000) ÷ Hours
• Severity Rate = (Days lost or charged × 1,000,000) ÷ Hours
• Average Days per Case = Severity Rate ÷ Frequency Rate
• 200,000 hours = 100 workers × 40 hrs × 50 wks
• 2,000 hours = one full-time worker-year
• EMR 1.0 = average; below 1.0 = better; above 1.0 = worse
• Source documents: OSHA 300 Log, 300A Summary, 301 Incident Report; benchmarks from BLS by NAICS code
Master the constants, read the numerator and the hours carefully, and treat every question as a short three-step calculation: count, exposure, constant. With that discipline, this topic becomes one of the most reliable scoring areas in the Safety Management domain.
Safety Management is one of the core domains of the CMAA Certified Construction Manager (CCM) exam, and the construction manager is expected to measure safety performance, not just talk about it. Incidence, frequency and severity rates are the standardized statistics that allow a CM to:
• Prequalify and compare contractors of very different sizes on an equal footing
• Benchmark a project or a company against industry averages published by the U.S. Bureau of Labor Statistics (BLS) and OSHA
• Track trends over the life of a project and trigger corrective action when the numbers move in the wrong direction
• Report to owners, insurers and regulators in a language they recognize
• Understand how a contractor's safety history drives its insurance cost through the Experience Modification Rate (EMR)
Because these are formula-driven, they are among the most predictable calculation questions on the exam. A candidate who memorizes two constants and understands what each rate measures can reliably collect these points.
2. What the Rates Are
All three rates share the same idea: take a count of something bad (cases or lost days), divide by the exposure (hours worked), and multiply by a constant so the answer is expressed per a standard-sized workforce. This normalization is what lets a 20-person subcontractor be compared with a 5,000-person general contractor.
A. OSHA Incidence Rate (also called Total Recordable Incident Rate, TRIR, or Recordable Incidence Rate)
Incidence Rate = (Number of OSHA recordable injuries and illnesses × 200,000) ÷ Total employee hours worked
• The constant 200,000 represents 100 full-time employees working 40 hours per week for 50 weeks (100 × 40 × 50 = 200,000). The result is therefore "recordable cases per 100 full-time workers per year."
• Recordable cases come from the OSHA 300 Log and include: fatalities, days away from work, restricted work or job transfer, medical treatment beyond first aid, loss of consciousness, and other significant injuries or illnesses diagnosed by a licensed health care professional.
• Variants using the same 200,000 base: DART rate (Days Away, Restricted or Transferred cases) and LTIR / Lost Time Incident Rate (cases with days away from work only).
• For reference, the construction industry TRIR published by BLS has generally been in the range of roughly 2.5 to 3.0 in recent years; a contractor well below that figure is performing better than average.
B. Frequency Rate (traditional ANSI Z16.1 method)
Frequency Rate = (Number of lost-time (disabling) injuries × 1,000,000) ÷ Total employee hours worked
• The constant 1,000,000 expresses the result as disabling injuries per million hours worked.
• Only lost-time injuries (not every recordable) are counted. This older method is still widely used by contractors, insurers and in CMAA study material, so you must know it alongside the OSHA method.
C. Severity Rate
Severity Rate = (Total days lost or charged × 1,000,000) ÷ Total employee hours worked
• Measures how serious the injuries were, not how many. One fatality or one permanent disability can produce a very high severity rate even when the frequency rate is low.
• "Days charged" includes scheduled charges for permanent injuries (for example, ANSI Z16.1 assigns 6,000 days to a fatality or permanent total disability).
• Some organizations compute severity on the OSHA 200,000-hour base (days lost per 100 workers). If an exam question states the base, use the base the question gives you.
D. Average Days per Lost-Time Case
Average Severity = Total days lost ÷ Number of lost-time cases = Severity Rate ÷ Frequency Rate
This is a convenient check and a frequent follow-up question.
E. Experience Modification Rate (EMR)
Not a rate you calculate on the exam, but closely related. The EMR is set by the insurance rating bureau (NCCI or a state bureau) from three years of workers' compensation loss history, excluding the most recent year. An EMR of 1.0 is industry average; below 1.0 means better-than-average experience and lower premiums; above 1.0 means worse-than-average and higher premiums. Owners and CMs commonly set a maximum EMR (often 1.0) as a prequalification threshold.
3. How It Works in Practice
Step 1 – Gather exposure data. Total hours worked by all employees for the period, from payroll or certified payroll reports. Do not include vacation, sick leave or holiday hours; include overtime actually worked.
Step 2 – Gather case data. From the OSHA 300 Log and 300A annual summary: number of recordables, number of DART cases, number of lost-time cases, and total calendar days away or restricted.
Step 3 – Apply the formula. Multiply the count by the constant first, then divide by hours. This avoids rounding errors.
Step 4 – Interpret. Compare against the contractor's prior periods, the project target, the BLS/NAICS industry average, and the owner's prequalification criteria. Rising incidence with flat severity suggests many minor injuries (housekeeping, PPE, training issues); low incidence with a severity spike suggests a single serious event that warrants a root-cause investigation.
Step 5 – Act. The CM's role is to use the data in the project Safety Management Plan: adjust toolbox-talk topics, increase inspections, require corrective action plans, or in extreme cases recommend removal of a subcontractor. Remember that these are lagging indicators; a mature program pairs them with leading indicators such as near-miss reports, inspection scores, training hours and job hazard analyses completed.
4. Worked Examples
Example 1 – Incidence Rate. A contractor records 6 OSHA recordable injuries while its employees work 480,000 hours.
TRIR = (6 × 200,000) ÷ 480,000 = 1,200,000 ÷ 480,000 = 2.5 recordables per 100 full-time workers.
Example 2 – Converting headcount to hours. A project averages 50 workers for one year. Assume 2,000 hours per worker per year (40 hrs × 50 wks).
Hours = 50 × 2,000 = 100,000. If there were 2 recordables: TRIR = (2 × 200,000) ÷ 100,000 = 4.0.
Example 3 – Frequency and Severity. Over 500,000 hours, a subcontractor has 3 lost-time injuries totaling 90 days lost.
Frequency Rate = (3 × 1,000,000) ÷ 500,000 = 6.0 lost-time injuries per million hours.
Severity Rate = (90 × 1,000,000) ÷ 500,000 = 180 days lost per million hours.
Average days per case = 90 ÷ 3 = 180 ÷ 6 = 30 days.
Example 4 – Working backwards. A contractor reports a TRIR of 1.5 on 800,000 hours. How many recordables?
Cases = (TRIR × Hours) ÷ 200,000 = (1.5 × 800,000) ÷ 200,000 = 6 recordables.
Example 5 – Comparing contractors. Contractor A: 4 recordables in 200,000 hours (TRIR 4.0). Contractor B: 12 recordables in 1,200,000 hours (TRIR 2.0). Although B had three times as many injuries, B is the safer performer because its rate is half that of A. This is exactly why rates, not raw counts, are used for prequalification.
5. Exam Tips: Answering Questions on Incidence, Frequency and Severity Rates
Tip 1 – Memorize the two constants and what they mean. 200,000 = OSHA incidence/TRIR/DART (per 100 workers). 1,000,000 = traditional frequency and severity (per million hours). If the question says "OSHA" or "recordable," think 200,000. If it says "frequency rate," "severity rate" or "per million hours," think 1,000,000. If the question explicitly gives a base, use the one given even if it differs from what you memorized.
Tip 2 – Identify the numerator carefully. Incidence and frequency rates use a count of cases. Severity uses days lost or charged. Read whether the question asks about all recordables, lost-time cases only, or DART cases; the answer choices will often include a distractor computed with the wrong numerator.
Tip 3 – Make sure exposure is in hours. If the question gives workers, weeks or months, convert to hours first. Standard conversions: 2,000 hours per worker-year, 40 hours per worker-week. Watch for questions that give a project duration in months and a crew size; multiply crew × hours/week × weeks.
Tip 4 – Multiply before you divide. Computing cases × 200,000 first and then dividing by hours keeps the arithmetic clean and reduces rounding errors that can push you toward a wrong answer choice.
Tip 5 – Sanity-check the magnitude. A construction TRIR is usually a single-digit number (roughly 1 to 6). A frequency rate per million hours is often 5 to 25. Severity rates run into the hundreds or thousands. If your TRIR comes out as 250, you probably used 1,000,000 instead of 200,000, or forgot to convert workers to hours.
Tip 6 – Know the interpretation questions, not just the math. Expect items such as: "Which rate best measures how serious injuries were?" (severity); "Why are rates used instead of raw injury counts?" (to normalize for workforce size and allow comparison); "Which document is the source of recordable case data?" (OSHA 300 Log / 300A summary); "Are these leading or lagging indicators?" (lagging).
Tip 7 – Understand EMR direction. Lower is better. 1.0 is average. An EMR of 0.80 means premiums roughly 20 percent below the base; 1.25 means 25 percent above. EMR is calculated by the insurance rating bureau, not by the contractor, and it reflects a three-year window that excludes the most recent policy year.
Tip 8 – Know the CM's role. On a CMAA exam the "best" answer usually positions the CM as the party who monitors, reviews and reports safety performance and enforces the owner's prequalification criteria, while the contractor retains primary responsibility for means, methods and site safety. Do not choose answers that have the CM directing the contractor's safety means and methods.
Tip 9 – Watch for the average-days-per-case trick. Severity Rate ÷ Frequency Rate = average days lost per lost-time case. If a question gives you both rates and asks for average days per injury, do not recompute from scratch; just divide.
Tip 10 – Recognize limitations when asked. Rates can be distorted by underreporting, small hour totals (one injury on a tiny project yields a huge rate), differences in recordkeeping criteria, and the time lag between a hazard and a recorded injury. The textbook answer is to use them together with leading indicators and to evaluate trends over multiple periods rather than a single number.
6. Quick Reference Card
• TRIR / Incidence Rate = (Recordables × 200,000) ÷ Hours
• DART Rate = (Days Away/Restricted/Transfer cases × 200,000) ÷ Hours
• LTIR = (Lost-time cases × 200,000) ÷ Hours
• Frequency Rate = (Lost-time injuries × 1,000,000) ÷ Hours
• Severity Rate = (Days lost or charged × 1,000,000) ÷ Hours
• Average Days per Case = Severity Rate ÷ Frequency Rate
• 200,000 hours = 100 workers × 40 hrs × 50 wks
• 2,000 hours = one full-time worker-year
• EMR 1.0 = average; below 1.0 = better; above 1.0 = worse
• Source documents: OSHA 300 Log, 300A Summary, 301 Incident Report; benchmarks from BLS by NAICS code
Master the constants, read the numerator and the hours carefully, and treat every question as a short three-step calculation: count, exposure, constant. With that discipline, this topic becomes one of the most reliable scoring areas in the Safety Management domain.
Test mode:
CMAA CCM - Incidence, Frequency and Severity Rates Example Questions
Test your knowledge of Incidence, Frequency and Severity Rates
Question 1
In the OSHA-based injury statistics used to prequalify contractors and compare them with national averages, what does the DART rate measure?
Question 2
The bid documents for a municipal library disqualify any bidder whose recordable incidence rate exceeds 3.0. One bidder began operations six months ago and submits its log for that period. The log lists five entries. One is a first-aid-only treatment, and the other four are OSHA-recordable cases. The bidder's payroll records show 180,000 hours actually worked in the six months. What recordable incidence rate should the construction manager use in the screening?
Question 3
What does a contractor's OSHA recordable incidence rate of 3.0 indicate about its injury and illness experience?
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