I only know that Variance is $ SD^2 $ and do not understand how it comes into the picture and so do not know if this answer is correct. Can someone please confirm whether it is correct?
Question: A particular process has two independent sources of variation. The standard deviation of the first source is 3 units and the standard deviation of the second source is 4 units. The standard deviation of the process is which of the following?
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Answer: Find the variance of all standard deviations by squaring each (3^2 + 4^2 = 9 + 16 = 25), then take the square root of 25 which is 5. By squaring each individual standard deviation (SD), you get the variance. By adding up the variances then taking the square root of that sum, you can assign a SD for the process as a whole.