Calculus of Risk

The Calculus of Risk is a concept used in Negligence / Unintentional Delicts to determine if a duty of care has been breached. It is used to determine the reasonable man test.

Probablilty of Injury
As in Bolton v Stone, if the injury is not likely to happen, there is no negligence.

The importance of the probabilty of injury is highlighted in cases such as St George vs The Home Office. A prisoner who was prone to epileptic seizures (particularly during drug withdrawal, as he was) was assigned an upper bunk, and was injured after falling off during a seizure. Had the prisoner been assigned a lower bunk, the chance of injury would have been lower, and the case outcome different.

Gravity of Injury
This basically looks at how serious the potential injury is... The more serious the potential injury, the more that should be done.

This can be unique to the victim, in Paris v Stepney Borough Council, an employee with one eye, and thus at greater risk of total blindness was found to have a valid claim for negligence against his employer for not providing adequate eye protection from debris as it increased the seriousness of injury.

Value/Utility of the activity
"One must balance the risk against the end to be achieved; the saving of life and limb justifies the taking of risk"

- Lord Denning

The Value of the activity doesn't remove the requirement to meet the duty of care, but it can adjust the level of care required. A key example of this is in Gillfillan v Barbour, a police officer speeding through a red light in poor conditions found that whilst Gillillan was not required to meet the usual standard of care for a driver, he was required to meet the standard of care required for an emergency service driver (and in this case he had not met this requirement).

Practically and availability of precautions
The practicality of precautions can be a major factor, whilst proactive water cleaning in a busy station florist was seen as a practical response in Piccolo v Larkstock Ltd, a case arising from accident on a yet-to-be-treated icy road did not meet the requirement as roads needing to be treated in a priority order, and non main roads being lower on this priority list, was seen as reasonable as it is not practical to treat all roads.

Cost of precautions
The cost in avoiding the potential harm is examined in cases such as Collins v First Quench Retailing Ltd, where a person injured in the process of a robbery claimed negligence as neither security screens nor an additional staff member were present. Whilst the additional costs of screens was not seen as reasonable, as a second staff member was recommended by the police as a deterrent, this was seen as a reasonable expense to reduce the risk.

Normal practice within the subject area
Whilst it was said in ICL Tech v Johnston Oils that the following of generally accepted practices is not enough to protect the defender from a liability if it resulted in a readily foreseeable and avoidable risk; In Nugent v Glasgow City Council, the council dealing with potholes in line with a code of practice on potholes allowed them to escape liability for an injury caused in a low priority pothole.