With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

No credit card required

Basic Statistics

Statistics may be defined in very erudite ways, but fundamentally, it is the art and science of making a big bunch of numbers into a little bunch of numbers so you can understand stuff about the big bunch of numbers. A GIS can usually do statistics without even breathing hard. Here are some basic statistical measures, most of which you probably know:

Average

The mean or average: add up all the numbers in a set and divide by the number of numbers. Expressed as a formula the average, called x-bar here, because of the x with the bar over it on the left-hand side, looks as shown in Figure 6-8.

FIGURE 6-8 Formula for average of a set of numbers This formula scares some people. It shouldn’t. It simply says what was said in the preceding text. More specifically: Assign integers to each number, like 1to the first number, 2 to the second, and so on until you reach the last number, which we refer to as n. (If there were 12 numbers, n would be 12.) The numbers then are x1, x2, x3, . . . x12. So when we say xi and i = 7, we are referring to the seventh number, named x7. The symbol shown in Figure 6-9 means “take the sum of” whatever follows it. So in the case of the symbol in Figure 6-9, what follows it is xi. What values of i do you use? Those that run from 1 through n, namely, 1, 2, 3, . . . 12. Finally, the formula in Figure 6-8 says divide the sum by the number of numbers, ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

No credit card required