In a negative-feedback system is AB < 0 or is 1/(1 - AB) < 1?

[Seeking Science]

Is feedback negative when AB < 0 or when 1/(1 - AB) < 1? And, correspondingly, is feedback positive when AB > 0, or when 1/(1 - AB) > 1? I've found nothing but conflicting information on this.

 

Also, while species, the regulation of hormones, nutrients, and even entire populations are governed via negative feedback, how does one determine what A and B are? In a furnace, for example, governed via a traditional thermostat, this is clearly a negative-feedback system. But what is A in this case, what is B, and which equation quantifies it as a negative-feedback system? AB < 0? Or 1/(1 - AB) < 1?

Edited March 12, 2013 by DBKelley2