The average value of each category is independently calculated, based on the available data we hold. The average total cost might therefore be calculated based on a different sample from the sample that we use to calculate the average values of the sub-categories. For example, a ship-owning company may have provided data for the total crew cost, without providing data for the analysis of this cost into crew wages, provisions, crew other. In this case, the average values of the 3 sub-categories do not include any data from this ship-owning company, whereas the average total crew cost includes data from this company. Therefore, the sum of the crew sub-categories does not equal to the total crew costs. This applies to all categories and sub-categories.