The sensitivity of an instrument is the

1. smallest increment in the input that can be detected with certainty
2. largest input change to which the instrument fails to respond
3. ratio of the change in the magnitude of the output to the corresponding change in the magnitude of the input
4. closeness of the output values for repeated applications of a constant input

Sensitivity of an instrument is ratio of the change in the magnitude of the output to the corresponding change in the magnitude of the input.