The notion of norms in translation was introduced by an Israeli translatologist, the late Gideon Toury, in the 1970s. The term refers to regularities of translation behaviour within a specific sociocultural situation. In 1990, by way of thanking him for his early support of the natural translation hypothesis, I sent him a short article for publication in his journal Target asserting that there are also norms in interpreting (see Sources below). Today nobody would doubt that there are such norms, and they are among the things translation students have to learn. However, natural translators may not be aware of them. For instance, they may not be aware that according to present-day norms nothing they invent themselves should be added to a translation.
Another norm that natural translators probably breach more often is the one that prohibits adding to or omitting from the translation. But how often, and what factors does it depend on?
Gideon Toury. Wikipedia, 2019.
Brian Harris. Norms in Interpretation. Target vol. 2, no. 1, January 1990.
Gideon Toury 1942-2016. Source: Wikipedia.