**INJURY RATE CALCULATION CONFUSION**

Because of confusion, questions are regularly asked about calculating and interpreting injury rates. Is DIIR the same as DIIR, etc. Let us explain the calculations and the injury rate differences.

Note: Injury statistic calculations worldwide are calculated in accordance with ANSI Z16.1

Firstly, a Disabling Injury (DI) I or Loss Time Injury (LTI) is considered as an injury or occupational disease where the injured/ill person misses the next work shift because of the injury/illness or has suffered some bone damage (eg. A fracture).

The DIFR is a ratio related of an injury per million manhours worked.

= XX% (this gives a % incident rate)

The 200000 figure in this calculations is essentially the normal number of hours that a 100 workers work in a year

(ie. 21.6 days/month x 12 months x 8 hour day x 100 workers = 207360 manhours per year)

On this basis the DIIR ratio is related to a percentage (%), because of the 100 worker exposure period.

Secondly, the number of days lost (ie. Number of days off work) is used in the severity rate calculations. A fatality is counted as 6000 days lost or off work.

The 6000 days lost was estimated based on a worker working 300 days per year and being off work for 20 years because of the fatality or a total permanent disablement.

Where a fatal or permanent total disability (eg. blind in both eyes) is calculated as 6000 days off work, ….loss of hand at wrist or below elbow or leg below knee 3000days,….. foot at ankle 2400 days, …… loss leg above knee or arm above elbow 4500, for example.

Here the DISR is ratio is again related to the number of day off work per million manhours worked.

If this DII is to be use to determine the percentage of improvement between two DII indices, the square root of each DII must be performed before making the comparison. (eg. √DII = xx%)

Thirdly, you can adapt all these formulae for specific purpose statistic measurements, eg. An Incident Frequency Rate = (No. of incidents reported x 1000000)/total manhours worked, or for near misses reported, etc.

Here these incidents could include recording first aid cases & DIs, damage incidents, theft incidents, etc, that you may decide to include in the calculation

Safety Performance is relative, so you can calculate these rates to allow comparing performance month by month, annually, or on a rolling 12 month period basis, etc

Incident statistics are always historical, but you could adapt the rates to measure safety performance, eg. No. of safety talks per manhours worked, or the % safety talks, or the No. of audits &/or inspections per manhours worked or the % of audits, etc, for example.

Note: Injury statistic calculations worldwide are calculated in accordance with ANSI Z16.1

Firstly, a Disabling Injury (DI) I or Loss Time Injury (LTI) is considered as an injury or occupational disease where the injured/ill person misses the next work shift because of the injury/illness or has suffered some bone damage (eg. A fracture).

**1.****Disabling Injury Frequency Rate (DIFR)**= (No. of DIs x 1000000) divided by total manhours worked, for the period (month, year, etc) under review.The DIFR is a ratio related of an injury per million manhours worked.

**2.****Disabling Injury Incident Rate (DIIR)**= (No. of DIs x 200000) divided by total manhours worked= XX% (this gives a % incident rate)

The 200000 figure in this calculations is essentially the normal number of hours that a 100 workers work in a year

(ie. 21.6 days/month x 12 months x 8 hour day x 100 workers = 207360 manhours per year)

On this basis the DIIR ratio is related to a percentage (%), because of the 100 worker exposure period.

Secondly, the number of days lost (ie. Number of days off work) is used in the severity rate calculations. A fatality is counted as 6000 days lost or off work.

The 6000 days lost was estimated based on a worker working 300 days per year and being off work for 20 years because of the fatality or a total permanent disablement.

**3.****Disabling Injury Severity Rate (DISR)**= (No. of injured days off work x 1000000) divided by total manhours workedWhere a fatal or permanent total disability (eg. blind in both eyes) is calculated as 6000 days off work, ….loss of hand at wrist or below elbow or leg below knee 3000days,….. foot at ankle 2400 days, …… loss leg above knee or arm above elbow 4500, for example.

*(Ref: ANSI Z16.1)*Here the DISR is ratio is again related to the number of day off work per million manhours worked.

**4.****Disabling Injury Index (DII)**= (DIFR x DISR) divided by 1000, which allows a combined frequency & severity measure yielding an index of total injury experience. DII is purely a ratio.If this DII is to be use to determine the percentage of improvement between two DII indices, the square root of each DII must be performed before making the comparison. (eg. √DII = xx%)

Thirdly, you can adapt all these formulae for specific purpose statistic measurements, eg. An Incident Frequency Rate = (No. of incidents reported x 1000000)/total manhours worked, or for near misses reported, etc.

Here these incidents could include recording first aid cases & DIs, damage incidents, theft incidents, etc, that you may decide to include in the calculation

**5.****Cost Severity Rate**= (money loss x 1000000)/ total manhours worked, giving a cost per million manhours workedSafety Performance is relative, so you can calculate these rates to allow comparing performance month by month, annually, or on a rolling 12 month period basis, etc

Incident statistics are always historical, but you could adapt the rates to measure safety performance, eg. No. of safety talks per manhours worked, or the % safety talks, or the No. of audits &/or inspections per manhours worked or the % of audits, etc, for example.

**It is hoped that the Injury rate calculation & interpretation is now no longer confusing**.