For X-bar, R, or s charts, data can be organized
where each case is a unit or where each case is a subgroup.
- If each case is a unit with a subgroup identifier,
cases are assigned to a category according to the value of the identifier. Table 1 is
an example of this type of data organization. The data do not have
to be sorted by subgroup. A
BY variable (the subgroup identifier) is required to sort and aggregate
data and label the process variable.
- If each case is a subgroup, there are as many variables
as individuals within one sample. A sample identifier is not required.
When there is a sample identifier, it is used for labeling. Table 2 shows
this type of organization.
Table 1. Each case
is a unit for X-bar, R, and s charts
| Subgroup |
Length |
| 8:50 |
6.35 |
| 11:30 |
6.39 |
| 8:50 |
6.40 |
| 11:30 |
6.46 |
| 8:50 |
6.32 |
| 11:30 |
6.37 |
| 8:50 |
6.39 |
| 11:30 |
6.36 |
| ... |
... |
Table 2. Each case
is a subgroup for X-bar, R, and s charts
| Subgroup |
N1 |
N2 |
N3 |
N4 |
| 8:50 |
6.35 |
6.40 |
6.32 |
6.39 |
| 11:30 |
6.39 |
6.46 |
6.37 |
6.36 |
| ... |
... |
... |
... |
... |