There must be also some constraints. Each CSR they should be assigned to exactly one shift per day, that's something we already know. Also, a CSR may require a specific shift on a certain day. For example, a CSR may talk to the manager, say, suppose for the next month on the 20th day, I'm going to have a day off because I'm going to have my wedding ceremony, something like that. A CSR may schedule a day off in events. If that has been approved, then for that CSR on that day all you can do is to give him or her that shift. Each CSR should be assigned to at most one night shift, so that's something defined or determined by the manager level. There are some night shift, as long as a shift asks the CSR to work after 06: 00 PM, then that's a night shift. For example, suppose there's a shift that works from nine to 18, then that's not a night shift. If the start and the ending time is ten to 19, is 11 to 20, then they are called night shifts. In any case, you cannot ask one person to work on two nights for one week. The company says, we don't do that. There are also some other requirements. For example, there may be some requirements regarding some shifts in some specific weekdays. For example, when we talk to the company, they say every Thursday, they ask or they invite some external speakers to come to the call center to give some training. The speakers may talk about how to be a good phone receiver, how to solve customers' problems, how to use Excel, for example. Okay? Those kinds of training programs are always hold during night on Thursday. If that's the case, and you also have a schedule asking one CSR to work every night on Thursday, then that's not a very good idea. The company says the following. For example, a CSR can at most take two night shifts on Thursday in a month every week you can take at most one night shift and you cannot do that for always be on Thursday, something like that. You can take at most two. Also, for example, Monday morning is a very busy morning, right? So at most three CSRs, they may have a day off on each Monday. Otherwise, the company feel that they don't feel comfortable. Alright? You may have certain weekday, certain shifts that there are some requirements. You cannot have too many or too few number of CSRs taking that kind of shifts. There are also some requirements regarding CSRs. For example, at least one Manager must be there in every night shift. Also some people must be there, be more experienced, being able to determine some important things. For example, CSR with more than two years of experience in the call center should account for more than 50 percent of the total on duty CSR. What does that mean? That means you don't want to have all the CSRs being some new comers. You want some of them and enough amount of them being able to deal with some difficult things, be more experienced. All right? All of these are something mentioned by the companies. Actually there are more, but as a lecturer, I'm going to just illustrate some of them. But you may still understand that all these issues are real issues. Somehow we need to find a way to formulate to incorporate them into our mathematical model. Finally, the company would also say some soft constraints. That's very typical. When you talk to a company about, hey, what's the rules that a schedule need to satisfy? They always give you a lot. Some of them are actually nice to have because there's no way for one to have a perfect schedule. We always have some constraints that are nice to have. That means they may be violated if needed, but hopefully they are satisfied. For example, here, let me list two. Fairness among periods, the company hope the following thing is not going to happen. The company hopes that if we have five extra CSRs, in period one and only one extra CSRs in period two, there's no s here. In this case, I have five here extra, and I have one here extra. Then for period one, I feel very comfortable because if somehow a lot of customers jumps in, I have extra persons to take care of them. But for period two, I feel uncomfortable because I only have one extra person as a buffer. If I can somehow reallocate the persons, it would be better if I can have three CSRs in each period. Okay, so that's one thing. Well, that sounds good, that sounds good. But then the issue is, how may we write it down mathematically, right? That's not the whole story. There's also another issue that is fairness among CSRs. It would be good to have similar numbers of night shifts for every CSR. What does that mean? Suppose I have CSR A having four night shifts and having CSR P to have two night shifts, it would be better for both of them to have three night shifts, otherwise, CSR A would complain, right? In this case, basically, what we need to do is that we need to take care of these two, which or these two soft constraints. But also you may see that, well, the statement here is quite ambiguous. Unlike the previous hard constraints that we listed in the previous page. Lows hard constraints are more precise and somehow you know how to define them mathematically, how to express them mathematically. But here for this soft constraints, it would be better to have similar extra bla, bla, bla. It is good to have, then this is hard to be formulated. Similar somehow needs to be defined, right? Later we're going to talk about the mathematical model, again, is only paired for you to get an idea about how OR may be applied to practice. But also you will see that if we really want our models to be useful to the managers, these are issues to be satisfied. There must be some way for us to determine how to define this issue. As modelers, as operations researchers, we need to define this particular soft constraint. We need to determine what's the indicator for measuring these and that. We somehow need to have some positive sense so that our indicator makes sense. But we also need to be technical so that the indicators can be transformed into mathematical programs. That's why we spend in between those technical engineering persons and that those nontechnical managers, someone need to be the bridge. In the some sense, we operations researchers, we other bridge.