Sickness - Avg day lost and % time lost

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Hi I have created a report to work out average days lost and % time lost , but in order to do this I need the total number of working hours available (which I also have calculated). The problem is when I select only sickness as the type  it only then brings through the people that have sick days against them and discards the rest of the organisation. I need to bring through everyone when I select Sickness so that I can calculate the months Avg days lost and time lost. thank you