Chapters: Normal and Exponential random variables.   Q: The cross-secitonal area of plastic tubing for use in pulmonary resuscitators is normally distributed with mean of 12.5 squared mm and a standard deviation of 0.2 squared mm. When the area is less than 12.0 squared mm or greater than 13.0 squared mm, the tube does not fit properly. If the tubes are shipped in boxes of one thousand, how many wrong-sized tubes per box can doctors expect to find?

Features Features Normal distribution is characterized by two parameters, mean (µ) and standard deviation (σ). When graphed, the mean represents the center of the bell curve and the graph is perfectly symmetric about the center. The mean, median, and mode are all equal for a normal distribution. The standard deviation measures the data's spread from the center. The higher the standard deviation, the more the data is spread out and the flatter the bell curve looks. Variance is another commonly used measure of the spread of the distribution and is equal to the square of the standard deviation.