The maximum possible error in y is given by,

           ....................(7)

where and are the errors in quantities x₁ and x₂. If x₁ and x₂ are measurable quantities then and are the least counts of the instruments used to measure them.

4. Least Square Fit:

Suppose you are measuring two quantities x and y linearly related by

           y = ax+b....................(8)

The best estimates for the slope a and intercept b of the straight line are obtained as follows.