Reliability is an important index to evaluate the properties of chemical process measurement data.In practical application and theoretical analysis the method of taking average or weighted average of observed data as the measurement value of unknown parameter is the special case of least square unbiased estimation.The reliability of measurement data arising from unbiased measurement is corresponding to variance.Based on the Gauss-Markov theorem

the variance of unbiased measurement has a lower bound

so the reliability of unbiased measurement cannot be guaranteed.A method used to improve measurement reliability by biased estimation was proposed.Firstly

the equivalence between classical measurement process and LS was discussed.Secondly

a quantitative description method was proposed to evaluate the reliability of biased measurement data.Finally

the effect of biased parameter on reliability was studied.Numerical solution of optimal value of biased parameter was also given.Simulation results showed that the proposed method was always superior to unbiased measurement process in terms of reliability given the reasonable numerical range of biased parameters.