Note to men: a valid comparison point would be 'trying to mend a car - using a brown felt-tip'.

This may not be a particularly fair or valid comparison however.

Ultimately, a valid comparison between two telescopes must take into consideration more specifications, when a general measurement becomes obtuse.

Valid comparisons require that similar offences between jurisdictions be compared.

These standard rating definitions are designed to allow correct machine selection and valid comparisons between manufacturers to prevent them from misstating the performance of their machines, and to guide designers.