Home / Statistical Tools / Control Charts / Moving Range
Moving Range¶
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QXL Stat Tools Tab > Control Charts > Variables > Charts as Individuals > Moving Range
A Moving Range chart is one panel, Moving Range Chart, built from measurements taken one at a time. Each point is the largest minus the smallest of the observations in a short window of consecutive rows, a pair of neighbors by default, so the chart shows how much consecutive measurements differ rather than what they are. It is the lower panel of an I-MR chart drawn on its own, without the individuals panel above it.
The ribbon entry is Moving Range, but the dialog it opens is titled I-MR Chart, because one dialog serves the I-MR, Individuals, and Moving Range entries and its title follows the family rather than the variant. Only the worksheet differs: the sheet this variant writes is named Moving Range Chart. Every option in that dialog is described on the Options page.
When to use it¶
- Your measurements are continuous, and the process gives you one measurement per point in time: one reading per batch, one part per hour, one result per test run.
- There are no subgroups to form. This dialog has no Subgroups group at all, so each selected row is one observation and the moving ranges are formed from consecutive rows.
- You want the moving ranges on their own, with no individuals panel above them. If you want both panels, use the I-MR entry; if you want the upper panel on its own, use Individuals.
If the process gives you two or more measurements per point in time, use an Xbar-R or an Xbar-S chart instead.
How it works¶
Quantum XL takes the selected rows in worksheet order and plots, at each position, the range of the observations in a window that ends there. The window length is the Moving range length: box, which is 2 by default, so each plotted point is normally the difference between an observation and the one before it. The first point has no complete window and is left unplotted, and no window is ever formed across a blank row.
The lines on this panel are built from a within sigma estimated once for the whole chart (once per phase when the chart is split), using either the Average moving range, which is selected when the dialog opens, or the Median moving range. The center line is the moving range expected from a window of that length given that estimate, and the limits are fixed multiples of it. Every window is the same length, so all three lines are straight.
The lower limit is floored at zero, because a range cannot be negative. At the default moving range length and the default sigma multiplier that floor is active, so the panel has no usable lower limit and can only signal upward.
The individuals panel is still computed even though it is not drawn, so the whole dialog behaves as it does on an I-MR chart. Two consequences show up on this variant. The out-of-control rules that need a location panel, rules 5 through 8, are greyed out. And reference lines are drawn on the location panel only, so a reference line entered here never appears.
Two settings change what the limits describe rather than how they are computed. Estimate Limits From computes them from the first few points 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 Moving 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 Moving Range Chart panel, plotting each moving 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.
- 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.
When the analysis finishes, the Control Chart task pane opens on the right. See Create Control Charts for what it can and cannot change.