Luggage allowance

My daughter has just been to Paris and was a bit worried that the weight of her luggage was over the limit. My daughter is very slim and weighs much less than me yet we would have had the same allowance had I been travelling. This may have been asked before, but how do airlines allow for this when calculating TOW when they have no idea what their passengers are going to weigh? Do they just rely on average weights and hope they don`t get a rugby team turning up at checkin?

Yes, they use average. There is a story of an aircraft which was unaccountably over weight. Turns out that there was a convention of coin collectors on board, carrying their collections in money belts. Needless to say the story dates from before todays security!