# User Tip: Accuracy vs. Precision

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The accuracy of a measurement or calculation in Sketchpad refers to how close the measured value is to the ideal, “correct” value. The precision of a measurement in Sketchpad refers to the number of decimal places used when the value is displayed on the screen.

Don’t confuse accuracy with precision. If you are displaying distances precise to the nearest tenth, Sketchpad can accurately represent the sum of two lengths measuring 1.4 as

1.4 + 1.4 = 2.8

If your precision is set to round displayed values to the nearest unit, however, the sum may seem nonsensical:

1 + 1 = 3

The accuracy of Sketchpad’s measurements and calculations is determined by the full computational power of your computer. Initial computations are usually accurate to about 15 significant digits. For instance, the square root of 2 is stored internally as 1.41421356237310. Calculations based on these initial values may be less accurate, because errors in the accuracy compound across calculations.

The precision of displayed measurements is determined by your choice in Units Preferences. Sketchpad rounds calculated values to your chosen precision when displaying them.

See also:

Sketchpad’s Internal Mathematics

Using Values