Dynamic allocation is designed to recognise that installations can change substantially within a phase and these changes can impact their requirements for free allocation. It aims to ensure fair distribution of free allowances as the overall level of free allocation decreases over the phase. The level of an installations free allocation will be adjusted based on a two-year rolling average of activity level. If there is a variation of +/- 15% of the Historical Activity Level (HAL) then the level of allocation will be altered accordingly. Increase in energy efficiency is encouraged, so if activity levels fall by 15% due to energy efficiency measures, allocations should not be affected.