Let's use an example of a square block with a hole drilled into it. For simplicity, I am using only the X & Y coordinates. One corner of the block is the 0, 0 datum, and the hole is drilled at coordinates 1, 1. This is a
Cartesian coordinate system. This is the system in which a CMM operates and usually reports data in an x, y, z format.
Therefore, the hole location has an x coordinate and a y coordinate. It may vary less than 1 or greater than one. If you were to analyze the x or the y results individually, chances are good that you would see a normal distribution in each axis.
However, Position results convert the separate x, y coordinates into part of a quasi
Polar coordinate system. A true Polar coordinate system would provide both a radial distance and a polar angle for each measurement. Position results consider only the diametrical distance regardless of the angle. Therefore, a hole at (0.9, 0), (1.1, 0), (0, 0.9), or (0, 1.1) are all considered to have the same Position despite the fact that they are off location in four different directions. This usually results in a right skewed distribution.
Not only do you have to either perform a non-normal capability analysis, or transform the data to normal (not recommended), but you do not have the necessary information to improve the capability. To get that information, you have to study the x and y results separately in order to move the hole onto target.