Wikipedia:Reference desk/Archives/Mathematics/2017 March 1

= March 1 =

Activity network question
Suppose that there are two activities emanating from the source node, A and B. There is one activity C, that depends on A alone; one, D, that depends on B alone; and one, E, that depends on A and B. It isn't obvious to me how to use a dummy in this case, so what do I do?--Leon (talk) 19:50, 1 March 2017 (UTC)


 * Can you add some context, and say what your objective is? (I'm familiar with dummy variables, but only in the context of regression.) Maybe you could give a similar example in which you can show how a dummy is used. Loraof (talk) 17:34, 2 March 2017 (UTC)


 * An ASCII text diagram may help:

E                  ^ |                  ^                  / \            C <- A   B -> D


 * StuRat (talk) 18:16, 2 March 2017 (UTC)


 * Thanks, but that just restates what he said. I'd like to know what the OP's objective is and how he considers that dummies can be used in this context. Loraof (talk) 18:41, 2 March 2017 (UTC)


 * Correct. I was replying to the OP, not you, as shown by my indent level. BTW, I fixed your indent level to show that you were replying to me. StuRat (talk) 18:45, 2 March 2017 (UTC)


 * Sorry. Thanks. Loraof (talk) 21:55, 2 March 2017 (UTC)


 * I think in the particular precedence chart style the OP is referring to (there are many), each task is represented by the edge of a directed graph and each node represents a possible stage of completion. Say node 0 is nothing done, 1 is A done, 2 is B done, 3 is A and B done, 4 is A and C done, 5 is B and D done, and 6 is A, B and E done, with other nodes to be filled in as more tasks are added to make a complete diagram. Then A connects 0 to 1, B connects 0 to 2, C connects 1 to 4, D connects 2 to 5, and E connect 3 to 6. So far there is nothing to show 3 is connected to 1 and 2, so you need two dummy tasks, say X to connect 1 and 3, and Y to connect 2 and 3. Not my area of of expertise (the whole field is pretty woolly if you ask me), but hopefully that helps. --RDBury (talk) 03:50, 3 March 2017 (UTC)


 * It is the case that two dummy activities are needed, one connecting the end of A to the start of E and one connecting the end of B to the start of E. If there is no more work in the project, activities C, D and E will come to a common end node, i.e. nodes 4, 5 and 6 posited in RDBury's post will coalesce.→31.51.161.146 (talk) 17:45, 5 March 2017 (UTC)