u stands for the
number of nonconformities with varying sample sizes.
np-chart. This chart uses a constant sample size of attribute data, usually
greater than or equal to 50. This chart is used to chart the number defective
in a group. For example, a hardware component might be considered defective
regardless of the total number of defects in it. np stands for the number
defective.
p-chart. This chart uses a variable sample size of attribute data, usually greater
than or equal to 50. This chart is used to chart the fraction defective found in
a group. p stands for the proportion defective.
X and mR (XmR) charts. These charts use variable data where the sample size
is 1. This is the chart we use the most often. X stands for individual value and
mR stands for moving range. More on this later.
X-bar and R (XbarR) charts. These charts use variable data where the sample
size is small. They can also be based on a large sample size greater than or
equal to 10. X-bar stands for the average of the data collected. R stands for the
range (distribution) of the data collected.
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