Direct comparison of (left) the visibility function and (right) the closure phases of data obtained of the same source at virtually the same baseline length and angle but at different epochs. From top to bottom: data sets 1&6 of R Cnc, 2&4 of R Cnc, 9&11 of X Hya, cf. Table 1. Errors of the squared visibility amplitudes are in the range of 5–15%; errors of the closure phases are in the range of 5–15 deg.