The autocorrelation function of electrical field strengths for different boundary conditions (tuner positions) at a given spacial position is proposed in the IEC standard 61000-4-21 as a measure for the determination of the number of uncorrelated boundary conditions in mode-stirred chambers. Additionally, an upper limit for the autocorrelation coefficient is given for a fixed number N of measured tuner positions only. In this paper, we analyze an approach given in the literature that includes the treatment of different N, but still gives results that are inconsistent with the daily measurement practice in mode-stirred chambers. A slight modification of this approach is proposed that leads to consistent results. This paper gives critical values for the autocorrelation coefficients for any number of measured tuner positions based on a statistical analysis of the well known probability distribution of autocorrelation coefficients. The degree of determination and the significance level remain as free parameters that have to be established by the community. The authors propose values for these parameters that are consistent with the example given in the standard.