In taxonomy, the assigning of greater or lesser importance to one character, as compared to another character, according to its known or assumed value. The initial choice of which characters to use in a classification is in itself a positive weighting process, termed selection weighting. Characters considered unreliable and consequently rejected, perhaps because they show too much environmental variation, are said to be given residual or rejection weighting. Once the characters have been selected some may be ascribed additional importance if, for example, they are known to be good diagnostically in other groups. When such weighting is applied before the classification is drawn up it is termed a priori weighting. An example would be the placing together of two taxa that share the one character chromosome number but differ in a number of other characters. Alternatively a classification may be constructed in which all the chosen characters are considered of equal importance; this is the normal procedure in *numerical taxonomy
. When the classification has been erected it will then be apparent which characters correlate well with other characters. Such characters are given a posteriori weighting. This second procedure provides a method for obtaining unbiased correlations of visible morphological characters with other characters, e.g. chemical, cytological, or genetic characters, that are not immediately apparent.