A measure of the minimum perceptible change in quality. A one JND change is accurately detected 75 percent of the time; a three JND change is accurately detected 99 percent of the time. There is a large number of JNDs of difference between NTSC as it is now received in U.S. homes and HDEP. This difference decreases in ATV systems in a hierarchical order. Some feel that a large number of JNDs will be necessary for consumers to purchase new TV sets.