X  MOVING RANGE CHART
This another two-part chart used to monitor processes with variable data.
This is similar to the X-bar, R Chart, except  the number of items observed
at each instance is equal to 1.  In other words, the X MOVING RANGE CAHRT
tracks  how each process data point varies from one another.

X-BAR
A mean or central line represents the average value of the process.

X-BAR  R CHART
This is a two part Control Chart used to monitor processes with variable data.
The data used on these charts is reported  in small subgroups containing at least 2,
or at most 10, individual data readings.     2 is less than or equal to N which is less than or equal to
10.         _  The X chart tracks how the processes average varies from one point to another.
The R ( or range ) chart tracks the variation  that occurs within each subgroup.

XPS
EXTRUDES POLYSTYRENE; X-RAY PHOTOELECTRON  SPECTROSCOPY

XRD
X-RAY DIFFRACTION

XRF
X-RAY FLUORESCENCE SPECTROSCOPY