a. Did it make sense to start the project by looking at supplier invoices?
It made sense. Raw material was sent by suppliers, the company confirms that how many resources are able to produce from the supplier invoices. Moreover, the company also can compare the supplier by using the supplier invoices. It can help the company to find which supplier can fulfill its requirement at the lower cost. The uncertainty in the supply chain also will be lower down.

b. Did the choice of three sites over a four-month period represent a reasonable sample?
No, the sample was not reasonable. The total supply spends for these three sites totaled about $850,000. $590,000 spent for air transportation. There were 14 instances where the air freight bill was at least 10 times higher than the value of the item transported. The buying cost of raw material is lower than the shipping cost because of remote suppliers.

c. What additional information would be helpful?
How urgently the parts were needed and what a reasonable downtime is. It can help the company to forecast the amount of items and delivery time accurately. Moreover, it can reduce uncertainty of remote supplier.

d. Is it possible to draw some tentative conclusions on this project?
The critical problem of this case is the air transportation cost. Alice Winter need to close with the suppliers and shorter the delivery distance. Besides this, the communication between the site leaders and suppliers is important. Make sure that the accuracy of communication is high. It can avoid the delivery delay and mistake.