Two metrics are discussed in this paper.
1. for 202 suppliers, Ford never maintained inventory with the result that any supply / supplier disruption immediately impacted the Ford assembly line.
2. There were 14 suppliers for whom Ford maintained one week worth of inventory and hence the TTS for those 14 suppliers were one week.
3. There were 147 suppliers for whom any disruption in supply would not matter much as Ford had inventory stocks of more than 52 weeks (a year) with it.