You seem to be talking about standard deviation. It is a widely used measure of quality for repetitive measurements, calculated with the following formula: $$\sigma=\sqrt{\frac1 N\Sigma(x_i-\mu)} ,\ where\ \mu=\frac1 N\Sigma x_i$$
For a known \$\sigma\$ you can expect 99% of your measurements to be within [+3\$\sigma\$; -3\$\sigma\$] interval. If your application requires to distinguish between values which are within 3\$\sigma\$ from each other or less, you may need a more precise sensor or some sort of noise cancellation algorithm.