Question: how to calculate the standard deviation of two variables?
Answer: The standard deviation formula may look confusing, but it will make sense after we break it down. ... 
Step 1: Find the mean.
Step 2: For each data point, find the square of its distance to the mean.
Step 3: Sum the values from Step 2.
Step 4: Divide by the number of data points.
Step 5: Take the square root.