For the graph to represent your data most effectively, consider the following guidelines.

- If possible, the x- and y-values should be spaced regularly to form a grid
- Usually, the z value is the response that you want to explain or predict and the x and y values are the explanatory variables. Minitab plots response (Z) values at the x-y intersections of the grid, or mesh. If your x- and y- values are not evenly spaced, Minitab must interpolate (estimate) the z-values at the intersections of a regular mesh with the same x- and y-ranges as your data. For more information, go to Working with mesh on contour plots and 3D graphs.
- The sample data should be selected randomly
- In statistics, random samples are used to make generalizations, or inferences, about a population. If your data were not collected randomly, your results may not represent the population.