From the course: Excel: Statistical Process Control

The normal distribution - Microsoft Excel Tutorial

From the course: Excel: Statistical Process Control

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The normal distribution

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- [Instructor] The values in most data sets fall into a familiar pattern with many more values close to the overall average and fewer values as you move away from the average. This pattern is called the normal distribution. And in this movie I'll describe the normal distribution and provide an example to make the pattern clear. Data that is normally distributed follows a specific pattern and that is called the Bell Curve or the normal curve. You can see here that there is an average and that along the bottom, there are standard deviations. The standard deviation is a measure of how spread out data is within a distribution. One of the properties of data that is normally distributed is that 68% of all the values will fall within one standard deviation of the average for the mean. 95% will fall within about two standard deviations and 99.7% will fall within three standard deviations. You can see that any values that are more than three standard deviations away from the average are exceptionally rare. And, should be considered exceptions to be investigated. To give a practical example, we can see individuals distributed by height and this data set is for American males aged 20 to 29. The average height for that age group in the U.S. is five feet 10 inches tall and there is a three inch standard deviation. So, you can see that at five foot seven, you're one standard deviation below the mean. And, at six foot one, you're one standard deviation above. And, 68% of American men within the ages of 20 to 29 fall within that range. 50% of individuals are going to be taller than average. That, I think, speaks for itself. And then, to the left of the average, we will have 34% of men who are between five seven and five ten. And the reason that we know it's 34% is that we're looking at individuals who are average height to one standard deviation below average. That's half of 68% which is 34. A further 16% of American men in this age group are five foot seven or shorter. So, that means that men who are taller than me at five foot seven, although I'm not 20 to 29, but I've never been any taller, there are 34% who are five foot seven to five foot 10 and then 50% who are above average. So, 84% of American men are taller than I am at five foot seven.

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