Statistical Process Control (SPC) Charts?

1. display a measurement of every unit that has been produced.
2. are exclusively used for data following a binary distribution.
3. indicate to the process operator the quantity that they have produced.
4. display upper and lower limits for process variables or attributes, and are used to signal when a process is no longer in control.

Which of these variables is a nominal variable?

1. Years of education
2. Height
3. Satisfaction level
4. Blood group