 Students, now let us look at one more interesting model for vendor selection, that is the set covering location model. Set covering location model is applicable for the selection of the vendors. Now if you look at this model, we have a minimizing function, minimize z where what you need to minimize is the sum of y j's for any value of j, subject to different constraints. Now the constraint is about the assignment. The first constraint if you see is that the sum of y j's should be greater than or equal to one for all j's. What does it mean, y cannot be less than one, y1 cannot be less than one, vendor. Then j actually belongs to n k, j could be any number n. Then y j should belong to 0 1, assignment b, y would either be 0 or it would be 1 for different values of j, so these are the three constraints which you need to define for a set covering model. y j is defined as before in the simple plant location model and n is the set of vendors, who can supply item k. The objective function thus minimizes the number of vendors employed, the set covering method is minimizing the number of vendors. And the first constraint actually ensures that each item will be supplied by at least one vendor. y when you say the sum of y j should be greater than or equal to one for all values of j, what does it mean, it means there should be at least one vendor for that item. One vendor will supply at least one, it can be more than one, it can't be less than one. Then the second constraint actually represents the binary nature of the DC envelope, so y j belongs to 0 1, either 0 or 1, so assignment type of a problem, either the vendor is selecting or not. But at least one vendor should be selected and it can be more than one for the purchase of item. The second constraint is all about the binary nature of that assignment. Then the model presented above actually assumes that the vendors have no capacity limitation. Set covering model k, the thing is that there is no capacity included. We assume that the vendors are actually uncapacitated, their capacity is unlimited. So they do not address the quality of the supplied items from the vendors. In this model there is no variable for quality, so we are not taking it explicitly as the part of the model. The set covering model also assumes that the cost related to purchasing are the same for all the vendors. There is no purchasing cost involved in it. In a simple plant location model we had the purchasing cost which could differ from vendor to vendor. But here there is no purchasing cost involved in it, essentially it's just like the assignment of that vendor. So that is also a limitation for set covering model. So we assume that the purchasing cost for all the vendors are actually same. So in set covering model you can see that it's a simple model in which you actually look at the assignment of the vendors and you try to minimize the number of vendors while trying to satisfy most of the demand. But at the same time there are so many limitations in it, there is no purchasing cost or you assume that in this model that the purchasing cost for all the vendors are the same which in reality might not be the case. We also assume that there is the quality of the vendors are same which might not be the case and we also assume that they have like face fixed ordering cost associated with them which might not be the case as well. So there are limitations to it which can be modified. So set covering model can be modified and such variables could be added to it. But the basic model is actually essentially the assignment model, thank you.