Top panel: middle row of the power spectrum Fisher matrix F_ij for five realisations of the random sparse sampling. The main peak in the middle is the inverse error of the middle bin of the power spectrum 1 /σ_ii. Moving away from the main peak, each point represents the correlation between that bin and the middle one 1 /σ_ij. Bottom panel: same for regular (black) and average of the realisations of the random (blue) sampling. In the case of regular sampling, there are secondary peaks at specific scales, which is a consequence of the fixed position of the patches. On the other hand, for random sampling, there is no preferred scale and correlation is evenly distributed between all scales. The symmetric shoulders on the main peak, at k ≃ 0.037 Mpc^-1 and k ≃ 0.06 Mpc^-1, in the random case are due to the design of the random mask, which has been obtained by a reflection in the x and y plane for simplicity (see main text). Note that the y-axis is in log scale.