It depends what you are comparing: If you're comparing technologies, then you want to eliminate the variable of condition. If you are comparing the values or

It depends what you are comparing: If you're comparing technologies, then you want to eliminate the variable of condition. If you are comparing the values or efficiency of actual old fridges and new fridges - for example, to decide whether to replace the old ones - then it's false data to assume the old fridges are in the same condition as new ones.