  We review metrics to assess the radio quietness of sites used for radio
astronomy. Concise metrics are needed to compare candidate sites for new
telescopes, to monitor the quality of existing sites, and to design telescopes
to work well at a given site. Key points of assessment are the receiver dynamic
range required for the strongest interferers and the expected fraction of
spectrum available for sensitive astronomical measurements. We propose three
metrics: (1) total radio frequency interference power, (2)
interference-to-noise power ratio and (3) time-frequency occupancy. Box plots
of these metrics summarise large quantities of information, highlight expected
ranges of interfering signal properties, and aid comparisons of sites and other
factors of interest. We provide examples for Square Kilometre Array phase one
deployment in Australia based on measurements made for the selection of this
site. The Square Kilometre Array will be the largest radio telescope in the
world.
