Discovered by Abraham de Moivre when he noticed that many phenomena cluster around an average value and in so doing form a bell shaped curve. The heights of Canadians for examples cluster around the average height and if all heights were graphed a bell shaped curve would appear. The normal curve is a similar idea, and has a similar shape, but is a theoretical curve (or one derived from mathematical manipulation rather than observation) and was developed by Friedrich Gauss to depict the effects of random variation. The normal curve is symmetrical so that if we draw a line from the highest point of the curve to the base, half of the curve will lie on one side and half on the other.