In statistics, numerical data is obtained through measurements and observations. To analyze and to be able to draw conclusions from data measurements a large number of method and tools can be used.
Some of the methods used to study date sets provide a single number that can be used to describe the data set. These include mean, median, and mode. These three are collectively referred to as measures of center. In addition to these there are also measures of spread. They provide a measure on how well centered or concentrated the measurements are. The measures of spread include range, interquartile range, and standard deviation.
Another way to analyze a data set is by describing how the data points are distributed. In this chapter the normal distribution will be studied. Data that is normally distributed is centered around the mean and its graph is symmetric and has a distinct bell-shape. To describe the spread of the data the standard deviation is commonly used. Data that typically is normally distributed include test results on standardized tests and birth weight of new born babies.
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