A calculated measurement in which the difference between the contractile force of a blood vessel in response to a specified dosage of norepinephrine and the baseline contractile force exerted by that vessel in the absence of any external stimuli, is divided by the baseline contractile force measurement and the result expressed as a percentage. Norepinephrine is a catecholamine which can act as a neurotransmitter, a stress hormone and a vasoconstrictor.