Scoring Severity of Injury — Hidden Probabilities

This entry is part 8 of 8 in the series Risk Assess­ment

I’ve been think­ing a lot about risk scor­ing tools and the algo­rithms that we use. One of the key ele­ments in risk is the Sever­i­ty of Injury. There are hid­den prob­a­bil­i­ties attached to the Sever­i­ty of Injury scores that are assigned that are not dis­cussed clear­ly in any of the risk assess­ment stan­dards that are com­mon­ly in use. This all start­ed when I was chal­lenged to write an analy­sis of the prob­lems with the CSA Risk Scor­ing Tool that you can find in the 2014 ver­sion of CSA Z434. That tool is deeply flawed in my opin­ion, but that is not the top­ic of this post. If you want to read my analy­sis, you can down­load the white paper and the pre­sen­ta­tion notes for my analy­sis from the Com­pli­ance inSight Pub­li­ca­tions page [1].

Scor­ing risk can be a tricky thing, espe­cial­ly in the machin­ery sec­tor. We rarely have much in the way of real-world data to use in the analy­sis, and so we are left with the opin­ions of those build­ing the machine as the basis for our eval­u­a­tion. Sever­i­ty is usu­al­ly the first risk para­me­ter to be esti­mat­ed because it’s seen as the “easy” one — if the char­ac­ter­is­tics of the haz­ard are well known. One aspect of sever­i­ty that is often missed is the prob­a­bil­i­ty of a cer­tain sever­i­ty of injury. We’re NOT talk­ing about how like­ly it is for some­one to be injured here; we’re talk­ing about the most like­ly degree of injury that will occur when the per­son inter­acts with the haz­ard. Let me illus­trate this idea anoth­er way: Let’s call Sever­i­ty “Se”, any spe­cif­ic injury “I”, and the prob­a­bil­i­ty of any spe­cif­ic injury “Ps”. We can then write a short equa­tion to describe this rela­tion­ship.

Se f (I,Ps)

Since we want there to be a pos­si­bil­i­ty of no injury, we should prob­a­bly relate these para­me­ters as a prod­uct:

Se = I x Ps

Ok, so what? What this equa­tion says is: the Sever­i­ty (Se) of any giv­en injury (I), is the prod­uct of the spe­cif­ic type of injury and the prob­a­bil­i­ty of that injury. More sim­ply yet, you could say that you should be con­sid­er­ing the most like­ly type of injury that you think will occur when a per­son inter­acts with the haz­ard. Con­sid­er this exam­ple: A work­er enters a robot­ic work cell to change the weld tips on the weld­ing gun the robot uses. This task has to be done about once every two days. The entry gate is inter­locked, and the robot was locked out before entry. The floor of the work cell has wire­ways, con­duits and pip­ing run­ning across it from the edges of the cell to the var­i­ous pieces of equip­ment inside the cell, cre­at­ing uneven foot­ing and lots of slip and trip haz­ards. The work­er miss­es his foot­ing and falls. What can you expect for Se in this case?

We know that falls on the same lev­el can lead to fatal­i­ties, about 600/year in the USA [2], but that these are most­ly in the con­struc­tion and min­ing sec­tors rather than gen­er­al man­u­fac­tur­ing. We also know that bro­ken bones are more like­ly than fatal­i­ties in falls to the same lev­el. About a mil­lion slips and falls per year result in an emer­gency room vis­it, and of these, about 5%, or 50,000, result in frac­tures. Ok, so what do we do with this infor­ma­tion? Let’s look at typ­i­cal sever­i­ty scale, this one tak­en from IEC 62061 [3].

Table 1 – Sever­i­ty (Se) clas­si­fi­ca­tion [2, Table A.1]

Con­se­quences Sever­i­ty (Se)
Irre­versible: death, los­ing an eye or arm 4
Irre­versible: bro­ken limb(s), los­ing a finger(s) 3
Reversible: requir­ing atten­tion from a med­ical prac­ti­tion­er 2
Reversible: requir­ing first aid 1

Using Table 1, we might come up with the fol­low­ing list of pos­si­ble sever­i­ties of injury. This list is not exhaus­tive, so feel free to add more.

Table 2 — Poten­tial Injury Sever­i­ties

Pos­si­ble Injury Sever­i­ty (Se)
Fall on same lev­el — Fatal­i­ty 4
Fall on same lev­el — Bro­ken wrist 3
Fall on same lev­el — Bro­ken col­lar­bone 3
Fall on same lev­el — Torn rota­tor cuff 2
Fall on same lev­el — Bruis­es 1
Fall on same lev­el — Head Injury 3
Fall on same lev­el — Head Injury 4

How do we score this using a typ­i­cal scor­ing tool? We could add each of these as line items in the risk reg­is­ter, and then assess the prob­a­bil­i­ty of each, but that will tend to cre­ate huge risk reg­is­ters with many line items at very low risks. In prac­tice, we decide on what we think is the most like­ly degree of injury BEFORE we score the risk. This results in a sin­gle line item for the haz­ard, rather than sev­en as would be the case if we scored each of these poten­tial injuries indi­vid­u­al­ly.

We need a prob­a­bil­i­ty scale to use in assess­ing the like­li­hood of injuries. At the moment, no pub­lished scor­ing tool that I know of has a scale for this, so let’s do the sim­ple thing: Prob­a­bil­i­ty (Ps) will be scored from 0–100%, with 100% being a cer­tain­ty.

Going back to the sec­ond equa­tion, what we are real­ly doing is assign­ing a prob­a­bil­i­ty to each of the sever­i­ties that we think exist, some­thing like this:

Table 3 — Poten­tial Injuries and their Prob­a­bil­i­ties

Pos­si­ble Injury (I) Sever­i­ty (Se) Prob­a­bil­i­ty (Ps)
Fall on same lev­el — Fatal­i­ty 4  0.0075%
Fall on same lev­el — Bro­ken wrist 3  5%
Fall on same lev­el — Bro­ken col­lar­bone 3  5%
Fall on same lev­el — Torn rota­tor cuff 2  5%
Fall on same lev­el — Bruis­es 1  90%
Fall on same lev­el — Head Injury 3 1%
Fall on same lev­el — Head Injury 4   0.0075%
Fall on same lev­el — Lac­er­a­tions to hands 2 90%

The per­cent­ages for fatal­i­ties and frac­tures we tak­en rough­ly from [1]. Ok, so we can look at a table like this and say that cuts and bruis­es are the most like­ly types of injury in this case. We can either decide to group them for the over­all risk score, or we can score each indi­vid­u­al­ly, result­ing in adding two sep­a­rate line items to the risk reg­is­ter. I’m going to use the oth­er para­me­ters from [2] for this exam­ple, and devel­op an exam­ple risk reg­is­ter, Table 4. In Table 4,

Se = Sever­i­ty

Pr = Prob­a­bil­i­ty of the Haz­ardous Event

Fr = Fre­quen­cy and Dura­tion of Expo­sure

Av = Pos­si­bil­i­ty to Avoid or Lim­it Harm

The algo­rithm I am using to eval­u­ate the risk is R = Se x [Pr x (Fr + Av)] [1]. Note that where I have com­bined the two poten­tial injuries into one line item (Item 1 in the reg­is­ter), I have select­ed the high­est sever­i­ty of the com­bined injuries. The less like­ly sever­i­ties, and in par­tic­u­lar the fatal­i­ties, have been ignored. You can click on  Table 4 to see a larg­er, more read­able ver­sion.

Table 4 - Example Risk Register
Table 4 — Exam­ple Risk Reg­is­ter

Note that I did not reduce the Se scores in the Final Risk Score, because I have not made changes to the slip/trip and fall haz­ards, only to the like­li­hood of the injury occur­ring. In all cas­es, we can show a sig­nif­i­cant risk reduc­tion after mit­i­ga­tion. I’m not going to get into risk eval­u­a­tion (i.e., Is the risk effec­tive­ly con­trolled?) in this par­tic­u­lar arti­cle, but the fact that you can show a sig­nif­i­cant risk reduc­tion is impor­tant. There are lots of con­sid­er­a­tions in deter­min­ing if the risk has been effec­tive­ly con­trolled.


Con­sid­er­a­tion of the prob­a­bil­i­ty of cer­tain kinds of injuries occur­ring must be con­sid­ered when esti­mat­ing risk. This process is large­ly undoc­u­ment­ed but nev­er­the­less occurs. When risk ana­lysts are con­sid­er­ing the sever­i­ty of injury from any giv­en haz­ard, this arti­cle gives the read­er one pos­si­ble approach than could be used to select the types of injuries most like­ly to occur before scor­ing the rest of the risk para­me­ters.


[1] D. Nix, ‘Eval­u­a­tion of Prob­lems and Chal­lenges in CSA Z434-14 Annex DVA Task-Based Risk Assess­ment Method­ol­o­gy’, 2015.

[2] Nation­al Floor Safe­ty Insti­tute (NFSI), ‘Quick Facts — Slips, Trips, and Falls’, 2015. [Online]. Avail­able: [Accessed: 21- Jul- 2015].

[3] ‘Safe­ty of machin­ery – Func­tion­al safe­ty of safe­ty-relat­ed elec­tri­cal, elec­tron­ic and pro­gram­ma­ble elec­tron­ic con­trol sys­tems. IEC 62061.’, Inter­na­tion­al Elec­trotech­ni­cal Com­mis­sion (IEC), Gene­va, 2005.


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