Left: fraction of zero-weighted data calculated according to Eq. (1) as a function of zenith angle and azimuth of a source. Each stripe corresponds to a single session. Pulsars were observed close to transit and the North celestial pole at the LOFAR core has a zenith angle of approximately 48°. Low-altitude observations are not necessarily more corrupted by RFI and the interference does not come from any preferable azimuth, though the statistics here are limited. Right, top: RFI fraction as a function of Universal Time (UT) and date. Colour coding is the same as on the left subplot. The RFI situation changes rapidly from one observation to another, likely due to the beamed nature of terrestrial signals. Right, bottom: fraction of zero-weighted data versus observing frequency in each of 400 sub-bands.