How do I test for normal distribution in Excel?

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While it’s possible to look up probabilities for a normal distribution using the z-table, it’s actually much easier to calculate probabilities in Excel for a couple of reasons. First, there’s no looking at a table; the NORMDIST function does the hard work for you. Second, Excel does the intermediate calculations for you. Most calculation errors happen in an intermediate step (such as calculating the z-score to look up) rather than the actual z-score itself. Excel can handle three types of probability calculations: more than, less than, and in between. These instructions work for Excel 2007 and Excel 2010.

Sample Question: Suppose that a contaminant in samples from a city’s water supply has a mean of 500 ppm and a standard deviation of 100 ppm. What is the probability that bacteria in a randomly selected water sample will be: 1/ less than600 ppm 2/More than 600 ppm 3/between 400 and 600 ppm?

Calculate Normal Distribution Probability in Excel: Less than

Step 1:

Click an empty cell.

Step 2:

Click “Insert Formula”.

Step 3:

Type “Normdist” into the search box and then click “Go.”

Step 4:

Select “NORMDIST” from the list and then click “OK” to open the Function Arguments window.

Step 5:

Enter your data into the box. For this example, type “600” in the X box, “500” in the Mean box, “100” in the Standard Deviation box and “true” in the cumulative box..

Step 6:

Click “OK.”. This returns 0.84134474 in the cell you clicked in Step 1, which is the probability of getting under 600 ppm.

Calculate Normal Distribution Probability in Excel: More than

Step 7:

Subtract your answer from Step 7 (above) from 1:
1-0.84134474= 0.158653.

Calculate Normal Distribution Probability in Excel: Between

Step 8:

Repeat Steps 1 through 8 for the second value, which for this example is 400. The probability is 0.15865526.

Step 9:

Subtract the larger probability from the smaller probability. In this example:
0.84134474 – 0.15865526 = 0.68269

That’s it!

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