In a Finish-to-Start dependency, does a successor activity have to start right after the completion of its predecessor? Not necessarily, it depends on the relationship they have!
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Gaining an Understanding
Precedence Diagrams help you to determine the project activity flow. Through the project activity flow, you can identify the critical path and compute the float of each activity. The schedule is created by using the Precedence Diagram and understanding the relationship between activities. I don’t mean the type of dependencies, such as Finish-to-Start, rather I mean the relationship between two dependent activities. The concept of Lead and Lag is critical in defining this relationship.
Regardless of the type of planning methodology or technique, such as Agile or Rolling Wave, the concept of Leads and Lags is applicable.
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Lead refers to a relationship whereby the successor activity begins before the predecessor activity has completed. For example, suppose you are baking a cake. As part of this, you will need to get the mixture ready and insert the dish into the oven. “Get the Mixture Ready” is the predecessor of “Insert the Dish into the Oven.” Pre-heat the oven is a task that is a part of the “Insert the Dish into the Oven” activity. Therefore, the “Insert the Dish into the Oven” activity should start before you’ve completed the “Get the Mixture Ready” activity. Assuming the pre-heating takes 20 minutes, then the “Insert the Dish into the Oven” activity should start 20 minutes before you have completed the “Get the Mixture Ready” activity. Therefore, the “Insert the Dish into the Oven” activity has Lead of 20 minutes.
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Lag refers to a relationship whereby the successor activity cannot start right after the end of its predecessor. For example, after you’ve baked the cake, you might want to serve it cold. Therefore, before serving it to the guests you will need to put the cake into a fridge and wait for it to cool. This means that the activity “Serve Guests Cake” will not start right after its predecessor “Insert the Dish into the Oven.” There is a delay. This delay is called Lag.
Here’s another example. Take a look at the following Precedence Diagram and identify the activity that’ll most probably have a Lag. (Click image for a larger view.)
If you identified the "Harden Foundation Cement" activity, then you are correct! Well done. There is a lag between "Lay Foundation" and "Harden Foundation Cement" because the latter activity would only start after the cement has matured. Only then can the construction workers start the "Harden Foundation Cement" activity.