Mean with Negatives

Dawn temperatures over four days were -7, 4, 0 and -5 degrees. The mean works exactly as before: add every value, sign included, then divide by the count. Adding -5 is taking away 5, so the line reads (-7 + 4 + 0 - 5) / 4. The top is -8, so the next line is -8/4, which is -2.

The zero counts. It adds nothing to the total, but it is still one of the four days, so you divide by 4, not 3. Leaving it out would give a colder and wrong average, as if that day had never been measured.

A negative mean makes sense here: the total is negative, because the cold days add up to more than the warm one. The mean always has the sign of the total, so check the total, not how many values are negative: -1, -1 and 10 are mostly below zero, yet their total is 8 and their mean is positive.