The Energy Efficiency Operational Indicator (EEOI) was developed by the IMO in order to allow ships to monitor the carbon emissions of their shipping activities. The EEOI is simply the total carbon emissions in a given time period per unit of revenue tonne-miles. The revenue tonne-miles accounts for the transport work done, measured in tonnes multiplied by the miles travelled carrying the cargo. Variations in the index are mainly caused by the technical efficiency of the ship, the amount of cargo transported per unit of time and variations in speed. However, as the EEOI is an aggregate number, it is difficult to disentangle the factors causing variation in the index. The objective of this note is to decompose the EEOI into ``sub-indices" of technical and logistical factors and compare the EEOI to alternative energy efficiency indicators that have been proposed at the IMO. Breaking down the basic formula leads to better transparency on the causes of variation of the EEOI. The sub-indices may help to improve operational and environmental performance for ship operators.