Sensitivity: Why Can Small Changes Become Amplified?

Sensitivity describes how much an output changes when an input, condition or parameter changes slightly. It is not the same as fragility. A system may be highly sensitive to one variable while still tolerating other disturbances. Nor is sensitivity identical to causal importance: a factor may be necessary without every small variation in it producing a large difference.

An audio mixing desk provides a simple example. Moving a fader a few millimetres through its middle range may change the volume gradually. Near the point where feedback begins, the same movement may make the sound suddenly harsh. Assessing sensitivity therefore requires us to specify which input is changing, which output is being observed and the range in which the change occurs.

Sensitivity is related to a threshold but is not the same concept. A threshold concerns whether a response begins after a boundary is crossed; sensitivity concerns the magnitude of change on either side of that boundary. The concept matters because a small cause need not produce a small result, but that claim becomes meaningful only when the amplifying relationship and its conditions are identified.