each drive manufacturer defines a set of attributes, and sets threshold values beyond which attributes should not pass under normal operation. each attribute has a "raw value", whose meaning is entirely up to the drive manufacturer (but often corresponds to counts or a physical unit, such degrees celsius or seconds), and a normalized value, which ranges from 1 to 253 (with 1 representing the worst case and 253 representing the best). depending on the manufacturer, a value of 100 or 200 will often be chosen as the "normal" value.