MASE frequency rate and severity rate: definition, calculation and best practices

Two acronyms keep coming up in exchanges with your local MASE association and in twice-yearly reporting files: TF and TG. The frequency rate and severity rate are the two results indicators that every certified company must produce and submit regularly. Their formula is simple on paper, but using them correctly — and above all understanding their limits — is less so. Here is how to calculate them without error, and why a good figure alone never convinces a steering committee on its own.

The frequency rate (TF): measuring how often accidents recur

The frequency rate expresses the number of lost-time workplace accidents per million hours worked. Its formula, taken directly from INRS methodology and used as-is within MASE, is as follows:

TF = (number of lost-time workplace accidents × 1,000,000) / number of hours worked

Two details change everything in practice:

• Only accidents that led to an effective time-off-work order (recognised “on first payment” by health insurance) enter the numerator. An accident without time off, or a near-miss without injury, is not added here — even though it must still be tracked elsewhere, in the monitoring indicators.
• The denominator uses hours actually worked, not theoretical headcount. A company whose workforce has varied significantly over the period (temp staff, subcontracting, one-off site work) must reconstruct this volume of hours rigorously — a rough approximation throws off the whole calculation.

The severity rate (TG): measuring the consequences of accidents

The severity rate complements the TF by weighting accidents by their severity, expressed as lost workdays from temporary incapacity per thousand hours worked:

TG = (number of lost workdays from temporary incapacity × 1,000) / number of hours worked

The numerator adds up the lost days from every accident over the period, not just the number of accidents: a single but severe accident (several weeks off work) can push the TG up far more than several minor accidents combined. That is exactly what the indicator is meant to reveal: two companies with the same TF can have very different risk profiles once the TG is factored in.

Calculating your indicators step by step: a worked example

Take a company that logged 250,000 hours worked over a rolling twelve months, with two lost-time workplace accidents resulting in 8 and 22 days of temporary incapacity respectively:

1. Number of lost-time accidents: 2
2. TF = (2 × 1,000,000) / 250,000 = 8
3. Cumulative lost workdays: 8 + 22 = 30 days
4. TG = (30 × 1,000) / 250,000 = 0.12

Taken in isolation, these two figures say nothing about the quality of a company’s SSE (health, safety, environment) management system. It is their evolution over time — from one half-year to the next, from one audit to the next — and how they compare with monitoring indicators that provide a useful reading.

Where these indicators fit within MASE

The TF and TG are part of the results indicators that the MASE reference framework requires companies to track, alongside monitoring indicators — near-misses reported, safety briefings, internal audits carried out — which measure prevention activity rather than its outcomes. This data feeds into the twice-yearly reporting sent to the local association, described in detail in our guide on renewing MASE certification: any anomaly or inconsistency in these submissions is escalated to the steering committee and weighs on the next audit.

One point deserves clarifying before any calculation: these rates are generally built on a rolling twelve-month period, recalculated at each half-yearly reporting deadline — not on a fixed calendar year. Check with your local association on the exact method used for your file, especially if your company’s scope has changed mid-cycle (merger, new activity), which must in any case be reported within one month.

Why a good TF is never enough: what the research says

The TF and TG are what the safety science literature calls lagging indicators: they measure an event after it has already happened. A reference study by Reiman and Pietikäinen, published in 2012 in Safety Science, points out that lagging indicators remain necessary to objectify outcomes, but that they must be complemented by leading indicators — an organisation’s capacity to detect accident precursors, the quality of safety management, effective reporting of near-misses — otherwise the company steers its prevention efforts purely by looking in the rear-view mirror (see the study). This is exactly the logic MASE follows by requiring, alongside the TF and TG, separate monitoring indicators: a steering committee that saw an excellent TF paired with zero documented safety briefings or internal audits over the period would have every reason to question how reliable that figure really is.

A figure your clients watch too

The TF and TG are not confined to the MASE file. Many clients in the industrial, petrochemical or maintenance sectors explicitly request them in tenders and prevention plans, independently of certification itself. A TF or TG that noticeably worsens year on year can therefore weigh on a tender well before any MASE audit takes place — which is why it pays to track these indicators continuously, not just as the half-yearly deadline approaches. Consistency between the figures submitted to your local association and those communicated to clients is itself a point of vigilance: a gap between the two datasets, even unintentional, undermines the credibility of the file.

Best practices for reliable tracking

• Document the calculation method used (source of hours worked, the definition applied to a “lost-time accident”) so it can be justified at audit without improvising;
• Recalculate at every half-yearly deadline, rather than reusing an old figure out of convenience;
• Systematically cross-check the TF/TG against monitoring indicators to avoid a good headline figure masking insufficiently active prevention;
• Analyse every accident, even minor ones, and keep a record of that analysis: it is this, more than the raw figure, that the auditor will want to review during the on-site audit.

Take action

Status, steps to obtain it, cost and frequently asked questions: find everything you need on MASE certification on our dedicated page, with a free ebook to help structure your process from start to finish.