Histogram Z Values

The following Statistical data is computed and displayed on the Histogram Chart. The data also is  available through exposed member functions. However, the data will get updated only when the Histogram is selected.

Z Values

Z values are the number of standard deviation units from the process average to a value of interest such as an engineering specification.

When used in capability assessment:

Z usl is the distance to the upper specification limit

Z lsl is the distance to the lower specification limit and

Z min is the distance to the nearest specification limit.

Zusl =  (USL – Mean) / Standard Deviation  -> P(d) LSL

Z lsl  =  (Mean - LSL) / Standard Deviation -> P(d) USL

Z Bench = from Z table

For Var QC

Z Bench:  P(d) Total  = P(d) LSL  + P(d) USL

For Att QC

Z Bench:  P(d)  = Total Number of defectives/ Total Number of samples


Median is the middle value in a group of measurements, when arranged from lowest to highest; if the number of values is even, by convention the average of the middle two values is used.  to The median is not an average.

More information

Histogram chart calculations.