what is a bell curve 
a bell curve gets its name from its appearance a bell shaped curve that rises in the middle it illustrates normal probability and several graphs and distributions use it the single line measures data on one two and three standard deviations 4the bottom linethe term three sigma points to three standard deviations shewhart set three standard deviation 3 sigma limits as a rational and economic guide to minimum economic loss around 99 73 of a controlled process will occur within plus or minus three sigmas so the data from a process ought to approximate a general distribution around the mean and within the predefined limits data that lie above the average and beyond the three sigma line on a bell curve represent less than 1 of all data points 