When vessels are involved in collision it is usually a result of failure to comply with the Collision Regulations or breach of the duty to exercise good seamanship. If both vessels are to blame they will in most countries be liable for the damage in proportion to their degrees of blame. A vessel claiming compensation for loss and damages will normally have the burden of proving that the other vessel has been negligent, in navigation or otherwise, and to fulfil that obligation the events leading up to the collision must be established as far as possible. For that purpose witnesses are examined and log books and other records on board are studied, although very often the facts are found to be in dispute between the parties involved. The reason may be that the statements of the witnesses are contradictory and impossible to reconcile, even if due allowance is given to the difficulties in making accurate estimates of time, distances, bearings, speed and course alterations. A closer examination of the observations made and the manœuvres carried out is therefore required.