Standard Deviation The Standard Deviation is a measure of how spread out numbers are. Its symbol is  σ  (the greek letter sigma) The formula is easy: it is the  square root  of the  Variance.  So now you ask, "What is the Variance?"     Formulas Here are the two formulas, explained at  Standard Deviation Formulas  if you want to know more: The " Population  Standard Deviation":               The “ Sample  Standard Deviation ”:           Looks complicated, but the important change is to divide by  N-1  (instead of  N ) when calculating a Sample Variance. When you have "N" data values that are: The Population : divide by  N  when calculating Variance (like ...

        In a normal distribution, 68% (34%+34%) of the results fall within one  standard deviation , and 95% (68%+13.5%+13.5%) fall within two standard deviations. At the center (the 0 point in the image above) the  median  (the middle value in the set), the  mode  (the value that occurs most often), and the  mean  ( arithmetic average ) are all the same.     Summary The lognormal distribution differs from the normal distribution in several ways. A major difference is in its shape: the normal distribution is symmetrical, whereas the lognormal distribution is not. Because the values in a lognormal distribution are positive, they create a right-skewed curve. The lognormal distribution model is considered to be very useful in the fields of medicine, economics, and engineering. Overall the log-normal distribution plots the log of random variables from a normal distri...

Difference between Descriptive and Inferential Statistics   Descriptive Statistics Use  descriptive statistics  to summarize and graph the data for a group that you choose. This process allows you to understand that specific set of observations Descriptive statistics frequently use the following statistical measures to describe groups:          I.           Central tendency : Use the  mean  or the  median  to locate the center of the dataset. This measure tells you where most values fall.      II.           Dispersion : How far out from the center do the data extend? You can use the  range  or  standard deviation  to measure the dispersion. A low dispersion indicates that the values cluster more tightly around the center. Higher dispersion signifies that data points fall further away from t...