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Surface and Contour 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 > Surface and Contour Plot.

  2. Tick both inputs. Each row carries a low and a high, filled in from the engine as a starting default.

  3. Run it.

What it plots

The equation, swept across a grid of the two inputs. It is not a simulation of the distributions: it evaluates the model at each grid point, so what you see is the shape of the formula itself.

B5 = B2*2 + B3 is a plane, so the surface should be flat and the contours should be straight, parallel and evenly spaced. Anything else on this model means the formula is not what you think.

Now make it curve

Change B5 to =B2*B3 and plot again. The surface becomes a saddle and the contours become hyperbolas, because a product is not a plane. That is the plot doing its job: showing the shape of the model at a glance.

The low and the high are yours

A box you have not touched sweeps the exact default, not the rounded number displayed in it: the rounding in the boxes is for display only. Type in a box and from then on it holds exactly what you typed, and nothing narrows, widens or clips it.

Things to try next

  • Tick a third input. Every unordered pair becomes a chart, so three inputs give three charts and the count grows quickly.
  • Set a low above its high and the dialog refuses by name rather than silently swapping them.
  • Compare with the Latitude Plot, which answers where the model stays in specification rather than what shape it is.

See Also