Concept
Weaver’s three levels
Three layers of communication problems: A technical (how accurately are symbols transmitted), B semantic (how well does meaning arrive), C effectiveness (how far does received meaning affect conduct).
Warren Weaver begins his 1949 essay by taking communication in a very broad sense (all the ways one mind may affect another) and lists problems at three levels. Level A, the technical problem: how accurately can the symbols be transmitted? Level B, the semantic problem: how precisely do the transmitted symbols convey the desired meaning? Level C, the effectiveness problem: how effectively does the received meaning affect conduct in the desired way?
Shannon’s mathematics belongs directly only to level A. Weaver argues that, since B and C can rely only on the accuracy that is possible at A, the limits at A also apply to them, and that the level-A theory can be counted, to a significant degree, as a theory for B and C too. This optimism was much debated later: Bar-Hillel and Carnap proposed a separate theory of semantic information in the early 1950s, while Shannon himself noted that carrying the theory over to other fields is an experimental task.
Level C can be read as Weaver’s counterpart of Lasswell’s “with what effect?” question; this is a mapping we make, not a sentence of the two authors themselves. Details: The Lasswell and Shannon–Weaver Models.
Example
Hypothetical example, a flood-warning SMS: garbled characters appear on screen (A); the word “evacuate” is misunderstood (B); the warning is understood correctly but nobody leaves home (C).