Secondary sources of strength, then, can be defined as controlling variables in which the response being analyzed is also controlled by a main thematic source. It also controls other response forms, and those other response forms would have a different effect upon the listener. Whether the relationship between the secondary source and the response is formal or thematic is irrelevant.
Controlling variables with the following features:
The response being analyzed is also controlled by a main thematic source
It also controls other response forms
The other response forms would have a different effect upon the listener
Whether the relationship between the secondary source and the response is formal or thematic