Shapes of Distributions
When there are many participants, the shape of a polygon becomes smoother and is referred to as a curve. The most important shape is that of the normal curve, which is often called the bellshaped curve. This curve is illustrated in Figure 7.2 on the next page. The normal curve is important for two reasons. First, it is a shape very often found in nature. For instance, the heights of women in large populations are normally distributed. There are small numbers of very short women, which is why the curve is low on the left; many women are of about average height, which is why the curve is high in the middle; and there are small numbers of very tall women. Here is another example: The average annual rainfall in Los Angeles over the past 110 years has been approximately normal. There have been a very small number of years in which there was extremely little rainfall, many years with about average rainfall, and a very small number of years with a great deal of rainfall. Another reason the normal curve is important is that it is used as the basis for a number of inferential statistics, which are covered in detail in Part D of this book.