How is an Inflection Point related to Sigma?
For a normal distribution, each inflection point is always one sigma away from the mean (mu). The graphic shows arrows going down from the normal curve to the horizontal axis (DV). Where that arrow touches the horizontal axis will be a value of the DV which is one sigma away from mu. It’s best just to look at the graphic and figure out the relationships before going on. The next paragraph talks through all the details, but grasping the big picture first will make the discussion easier. One sigma above mu. In our example we’ve let mu (mean) equal 200 and sigma (standard deviation) equal 25. As we’ve just said, the inflection points are one sigma away from the mean. Since sigma = 25, the upper inflection point (to the right, toward positive infinity) will be 25 units above 200 (mu). So the arrow cuts the axis at 225 (which is 200 + 25). The score 225 would be exactly one sigma above mu. 225 would be the value right below the upper inflection point. We add 25 (sigma) to 200 (mu) to get 22
