Originally Posted by

**ThatGuy** I'd like to add two normal distributions. It's 10 +/- 1 added to 10 +/- 1, with 96% of the cases contained within 9 to 11. Two sigma = sqrt ([1^2] + [1^2]).

It represents two 10 ohm resistors in series, which give a composite 20 ohm resistor with tolerance +/- 1.41 ohms.

I did this on a spreadsheet to avoid having to debug a program with nested loops, etc., and each normal curve is approximated by a histogram with 8 bins, with 20 boxes total equaling the area under the curve, which is 1, like so.

x

xx

xxx

xxxx

xxxx

xxx

xx

x

on the left is the value and on the right is how often it occurs.

9............ 1

9.285714286 2

9.571428571 3

9.857142857 4

10.14285714 4

10.42857143 3

10.71428571 2

11.......... 1

The problem is the extremes are off and so is the center value.

value...#.cdf

18.000 1 1

18.286 4 5

18.571 10 15

18.857 20 35

19.143 33 68

19.429 46 114

19.714 56 170

20.000 60 230

20.286 56 286

20.571 46 332

20.857 33 365

21.143 20 385

21.429 10 395

21.714 4 399

22.000 1 400

Any clues as to why? I'm most interested in the Z values that contain 50% and 96% of the cases.