Home / Statistical Tools / Control Charts / Xbar
Xbar¶
From Excel click...
QXL Stat Tools Tab > Control Charts > Variables > Charts in Subgroups > Xbar
An Xbar chart is a single panel built from measurements collected in subgroups. It plots the mean of each subgroup, with a center line at the grand mean and control limits computed from the variation inside the subgroups.
The dialog is not titled Xbar
Xbar is a variant of the Xbar-R chart, not a dialog of its own. Clicking Xbar on the ribbon opens a window titled Xbar-R Chart, with the Xbar only variant already selected, and the title stays Xbar-R Chart for as long as you are in it. Only the worksheet carries the name you clicked: the analysis writes a sheet named Xbar 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.
- You want the limits on the means to reflect only the variation inside a subgroup, so that a shift between subgroups shows up as a point outside the limits rather than being absorbed into wider limits.
- You want the panel of subgroup means on its own, without the range panel beside it.
If you want both panels, use an Xbar-R chart or an Xbar-S chart. If you want the dispersion panel on its own, the ribbon has separate R and S entries. 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 mean, and that is what the panel plots.
The center line is the grand mean of the individual observations. With unequal subgroup sizes that is not the same as the average of the subgroup means, because a larger subgroup carries more weight.
The limits come from a single within-subgroup standard deviation estimated once for the whole chart, using either Rbar (the default, built from the subgroup ranges) or the pooled standard deviation. Sbar is not offered on this dialog. The limits sit a number of standard errors either side of the center line, and the standard error uses each subgroup's own size: equal sizes give straight lines, unequal sizes give limits that step in where a subgroup is large and out where it is small.
The Range panel is still computed, it is just not drawn. Choosing the Xbar variant decides what appears on the worksheet, not what the engine works out, so the subgroup ranges are formed either way. Under the default Rbar estimator they are what the sigma behind your limits is built from.
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 Xbar 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 Xbar Chart panel, plotting each subgroup mean with its center line and control limits, plus the zone lines, reference 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 second panel below 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 Range 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 Xbar chart reports the Xbar panel only.
When the analysis finishes, the Control Chart task pane opens on the right. See Options for what it can and cannot change.