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: http://​nfsi​.org/​n​f​s​i​-​r​e​s​e​a​r​c​h​/​q​u​i​c​k​-​f​a​c​ts/. [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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Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. ( in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity.

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