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R¶
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QXL Stat Tools Tab > Control Charts > Variables > Charts in Subgroups > R
An R chart is a single panel built from measurements collected in subgroups. It plots the range of each subgroup, the largest value minus the smallest, against a center line and control limits that describe how much spread a subgroup of that size is expected to show.
The dialog is not titled R
R is a variant of the Xbar-R chart, not a dialog of its own. Clicking R on the ribbon opens a window titled Xbar-R Chart, with the R only variant already selected, and the title stays Xbar-R Chart for as long as you are in it. Only the worksheet carries a name of its own: the analysis writes a sheet named Range Chart.
Every option in that dialog is described on the Options page.
When to use it¶
- Your measurements are continuous, and you collect two or more of them per point in time: five parts an hour, three samples per batch, four cavities per shot.
- The subgroups are small. A range is an easy statistic to compute by hand and behaves well for a handful of observations per subgroup. For larger subgroups, the S chart uses each subgroup's standard deviation instead, which uses more of the information in the subgroup.
- You want the spread panel on its own, without the panel of subgroup means above it.
If you want both panels, use an Xbar-R chart. If you want the means on their own, the ribbon has a separate Xbar entry. If your process gives you one measurement at a time, use an I-MR chart instead.
How it works¶
Quantum XL divides your rows into subgroups: a constant number of consecutive rows, an identifier column whose repeated value marks a subgroup, or one subgroup per row built across several measurement columns. Each subgroup contributes its range, and that is what the panel plots.
A dispersion panel is not built the way a panel of means is. Its center line is the range expected from a subgroup of that size given a single within-subgroup standard deviation estimated once for the whole chart, so the center line steps when the subgroup sizes differ, and its limits are fixed multiples of that center line. The lower limit is held at zero, so on small subgroups the panel can only signal upward.
The sigma behind those lines comes from either Rbar (the default, built from the subgroup ranges) or the pooled standard deviation. Sbar is not offered on this dialog.
A subgroup with fewer than two usable values has no range, so that position on the panel is empty.
The Xbar panel is still computed, it is just not drawn. Choosing the R variant decides what appears on the worksheet, not what the engine works out.
Two settings change what the limits describe rather than how they are computed. Estimate Limits From computes the limits from the first few subgroups only and extends them across the whole chart. Split Control Limits divides the chart into phases, each with its own center line and limits.
The exact formulas are on the Math Details page.
Output¶
The analysis writes one worksheet per chart, named Range Chart. On it, from the top:
- A User Input block in the first two columns, holding the analysis title, the columns you selected, and whether the calculation type was Excel or GroupBy. Modify Existing Analysis reads it back.
- The Range Chart panel, plotting each subgroup range with its center line and control limits, plus the zone lines and phase dividers if you asked for them. Points that broke an enabled out-of-control test are drawn in a different color. There is no panel of means above it.
- A summary table to the right, listing the center line, the control limits where they exist, the count of out-of-control points, and the count of points you marked as outliers. A transformation adds rows naming the type and its parameters; a split chart adds a per-phase breakdown.
- A capability and histogram block below the panel, but only when you entered a specification limit or a target on the Capability tab.
The Xbar panel is computed but not drawn on this variant, and it gets no row in the summary table either. The table is filtered by the same setting that decides which panels are drawn, so an R chart reports the Range panel only.
The capability analysis, when you ask for one, is a separate matter: it is computed from your individual measurements rather than from the plotted ranges, so it is unaffected by which panel this variant draws.
When the analysis finishes, the Control Chart task pane opens on the right. See Options for what it can and cannot change.