The glass is half-empty, on average
Here's my answer to that age-old question: the glass is half-empty. Whether or not that makes me a pessimist is irrelevant, because my answer is based on statistics!
Whether or not a glass if half-full or half-empty depends on whether it is in the process of being filled, or emptied. If a glass is being filled, when it reaches half-way then it's half-full. If it's being emptied, when it reaches half-way then it's half-empty. Make sense?
So how do we know whether the glass is being filled or emptied? Well we don't. But we can work out the average based on (admittedly unscientific)...