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This lesson offers an example of calculating the critical path. For example, there are activities A,B and C; Activity C cannot happen until A and B are complete. In this lesson, the instructor uses the whiteboard to draw a diagram to demonstrate the process of calculating a critical path. In calculating a critical path; there are a few methods:
[toggle_content title="Transcript"] An example of calculating the critical path, as you know each one of these activities. You are going to say activity A, B, C, D, and E. Then, I will give duration to each one of these. Activity A could make up a number four, three, seven, two four. Then I will give an example of C requires B and A to be finished. Then I am going to say D requires A to be finished. E we say...E requires D and C to be finished. There are so many ways I can write this. All you are going to do is you are going to take this data and you are going to create a block diagram for this. What I will like to do is first knowing A is four days and then B can also be at the same level. B is three days. Next am looking at C. C requires A and B to be finished. I know to draw that like this. The duration for C is seven. D requires A to be finished...has nothing to do with B or C just A then it takes two days. E requires D and C to be finished so D and C are right here so and it requires to be finished. I am going to draw E right there and E takes four days. The first thing I want to do forgo how long this takes. I want to do a forward pass. I start of f in day one. A and b start off in day one. A is one plus four minus one. Remember, start off on a Monday so Monday Tuesday, Wednesday, Thursday. I am going to finish on a Thursday not on a Friday so you going to remember to subtract the one. B starts off in day one so Monday, Tuesday, and Wednesday. So it is one plus three minus one. In between tasks if it ends on the third day, I have to start on the fourth day. C requires both of these to be complete so I take the higher number. This is four plus one, which gives me five. D requires only A to be complete so four plus one is still five. D takes two days so five plus two minus one gives me six. C takes seven days five plus seven minus one. What would that be eleven? E requires both C and D to be complete in order to start so I have to go the higher number. It is going to start on the twelve day. Eleven plus one is twelve. E takes four days minus one so fifteen. That is the forward pass so to complete this project I know it is going to take fifteen days. Now I am going to give the backward pass. What the backward pass allows me to do is forgo slack. If it's on the critical path, we will have a little room in case anything goes wrong. I want to take this fifteen so doing the opposite. So it's fifteen minus four plus one. Good way to checking yourself is if you know you are on the critical path this numbers, fifteen minus fifteen is going to be zero. Twelve minus twelve is going to be zero. This twelve, I am going to subtract one I come up with eleven. If you notice D, it is six here but am transferring this to eleven so D is not on the critical path which make sense if its only taking two days unless it is taking seven days. Eleven minus two plus one is ten, eleven minus seven plus one should be five. I am going to take the lowest number and subtract one so this will be four. Doing the same method four minus four plus one gives me one. Four minus three plus one gives me two. I can actually start B on the second day. I don't need to start on the first day. The way I found out that is I take...doesn't matter which one this minus this or this minus this. That gives me one day of slack. This is on the critical path so there is zero slack. For this, one doesn't matter which sign I say ten minus five gives me five. So I have five days of slack there and this is on the critical path. Sometimes I ask you what critical path is, and you are looking for that zero slack, I would say A-C-E-ACE. That would pass me one of the multiple choice answers for this question or I might ask you would is the slack for D, or I might give where the problem is...what is the latest D can start on in order for the schedule. [/toggle_content]