A precedent for the definition of the phrase was set by the Court of Appeal when considering the case of Edwards -v- National Coal Board (1949). The plaintiff was the widow of a coal miner killed underground by the collapse of a roadway on which he was walking. The duty holder must weigh the severity of the risk against the cost of implementing measures to avoid or reduce the risk. If the costs are considered disproportionate in comparison to the risk likelihood, then it is unreasonable to expect the duty holder to implement the measures. But the situation should be kept under review, as changes in the severity of the risk or in the cost of control measures may alter the comparative weighting.