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Latitude Plot How-To

This walkthrough uses a small model you can build in a minute: two inputs, one output, and a specification band that half the output sits outside of.

Press Copy for Excel, then paste the table into a blank worksheet.

Model Value
Wall thickness 10
Bore 4
Outside width =B2*2+B3

Click cell A1 in a blank worksheet and press Ctrl+V. The two headings land in row 1, the labels in column A, and the model in column B: B2 holds the wall thickness, B3 the bore, and B5 the formula that reads them both. The empty row is deliberate, so the output lands in B5 and not B4, which is what every Monte Carlo walkthrough calls it.

Mark B2 and B3 as inputs with Mark Input, giving each a Normal distribution: B2 mean 10 standard deviation 0.2, B3 mean 4 standard deviation 0.1. Mark B5 as an output with Mark Output and give it a lower limit of 23 and an upper limit of 25.

Steps

  1. QXL Monte Carlo New > Monte Carlo > Contribution Tools > Latitude Plot.

  2. Each input row carries Low, High and Nominal. They arrive filled in from the marked distributions; leave them for this first plot.

  3. Run it.

What the plot shows

Where the pair of inputs keeps every output inside its limits. The plotted region is the set of settings that stay in specification; outside it, at least one output does not.

The output's limits are what define the region, so a plot needs an output with at least one specification limit. B5 has 23 and 25, which is why this model produces one.

Read it against the arithmetic

B5 = B2*2 + B3, so B5 sits between 23 and 25 when 2*B2 + B3 does. That is a straight line in the two inputs, so the in-specification region should be a band with straight edges running across the plot. A curved or broken edge on a model this simple means the markings are not what you think.

Things to try next

  • Widen B2's Low and High and the band's position on the plot stays put while more out-of-limit area appears around it. The region is a property of the model, not of the window you view it through.
  • Tighten B5's limits to 23.5 and 24.5 and re-run. The band narrows, which is the sensitivity you came for.
  • Tab into a Low, High or Nominal box. Its contents are selected, so typing replaces the value.

See Also