Bragg’s Law λ = 2dsinθ
Precision in ‘d’ is a function of sin θ, and cot θ.
Differentiating Bragg’s law with respect to θ, we obtain
Other relationships user:
The lattice parameters obained is plotted against cos^{2}, cos^{2}θ/sinθ and the NelsonRiley function
an the extrapolation of the curves to cos^{2 }= 0 will give true value of lattice parameter, i.e., a_{o}
By this process two different errors may arise:
(i) 
Large systematic errors, small random errors

(ii) 
Small systematic errors, large random errors

Note:
(i) Systematic errors can be reduced by proper extrapolation function
(ii) Random errors can be reduced by Cohen's method (for further information about cohens method see Cullity text book)