Normal distribution

The normal distribution is also known as the bell curve. A population that has a normal distribution can be describe by the mean and standard deviation (the variability, spread, or dispersion of values) of the population. Excel functions automate many of the calculations necessary when working with normal distribution. Given the mean, std. dev. and a value in the distribution, the function =normdist will give you the proportion of the population that has lower values than that given. For example, IQ scores are normally distributed with a mean of 100 and a standard deviation of 16. The syntax of the normdist function is normdist(x(value of interest),mean,standard_dev,cumulative(always us a "1" for this attribute). The proportion of the population with IQ scores less than 110 is =normdist (110,100,16,1) = .734. Of course, 1-.734= .266 is the proportion above IQ 110.

norminv is the reverse operation of normdist. That is, given a mean and standard deviation, norminv will give the value in the population that has some given proportion less than that value. An example will make it clearer. Mensa in an organization for very intelligent people. You are supposed to be in the top 2% of IQs to join. What IQ is that? We need to find the IQ value that has 98% of the population less that that IQ value. Use =norminv(probability (less that the value),mean,standard_dev). In this example we get =norminv(.98,100,16) = 133. By this calculation we can conclude that only 2% of the population have IQ scores greater than 133.

If you need more info use Excel help or do a web search on the functions.