The hierarchy of controls is based on the concept that not all risk treatments are equally effective. For example a handrail at the top of a cliff (engineering solution) is going to be more effective than putting up a sign to stay away from the edge of the cliff (an administrative risk treatment).
In order of priority it is best to:
Eliminate the risk
Substitute the source of the risk
Isolate the resources from the risk
Engineer barriers to protect from the risk
Administrative controls to protect resources
Protective equipment when exposed to the risk
In many risk settings, the objective is to render the risk ALARP. This stands for As Low As Reasonably Practicable. You can find out more about ALARP in this article on my main website.
The term has been enshrined in UK case law since the case of Edwards v. National Coal Board in 1949.The ruling was that the risk must be significant in relation to the sacrifice (in terms of money, time or trouble) required to avert it: risks must be averted unless there is a gross disproportion between the costs and benefits of doing so.
As Low as Reasonably Practicable (ALARP) is a level of risk that is tolerable and cannot be reduced further without expenditure of costs disproportionate to the benefit gained or where the solution is impractical to implement.