You need a measure of variation for your Green Belt project. Since you are not sure if you have normal or non-normal data, which of the following measures should you choose? Question 27 options: a) range: the high value minus the low value b) variability ratio (VR): the high and low numbers are used to calculate the interquartile range (IQR) in the PVR c) standard deviation: the statistical index of variability that describes the spread; also called sigma d) all the above