Jacksonville is a city in the U.S state of Florida. The settlement in Cowford as earlier known began in the year 1791 and the city received its present name. in 1822 in honor of Andrew Jackson, the first military governor of the Florida Territory. In 1821, Florida became a territory of the United States. The city grew rapidly and was chosen by the voters to consolidate the city and county governments in 1968. Thus Jacksonville was made the largest city in land area in United States. It is now the county seat of Duval County.
First, you have to know a little about statistics. I assume that the grades are drawn from a normal (bell-curve) distribution. These distributions have an average (also called a mean) and a standard deviation (stdev), which measures how wide the distribution spreads out.
Standard Deviation Plot. Standard deviation plots can be used with ungrouped data to determine if the standard deviation is changing over time. In this case, the data are broken into an arbitrary number of equal-sized groups. For example, a data series with 400 points can be divided into 10 groups of 40 points each. A standard deviation plot can then be generated with these groups to see if the standard deviation is increasing or decreasing over time. Although the standard deviation is the most commonly used measure of scale, the same concept applies to other measures of scale.