İn a workschedule, activity relationships must model the nature of work which will be performed. In other words activity relationships must model the reality. The main purpose of scheduling is to create a living and dynamic model capable of representing the real work.
Activity relationships, lag and leads are used to represent the nature of work. FS, SS, SF and FF are 4 relationships but sometimes people may use complicated relationships instead of activities. In order to avoid mistakes in project scheduling, lead and lag time should be known correctly.
Lead and Lag Time
Lead is only found activities with Finish-to-Start relationships. Lead means the beginning of a successor activity before the completion of its predecessor. Lead is the acceleration of the successor activity or simply we can say that lead is the overlap between the predecessor and the successor activity. Lead time can be considered a negative value.
For example ,you are a procurement manager of a steel bridge project and you are planning manufacturing and transportation activities. In your initial plan, relationship between the manufacturing and the transportation activity is Finish-to-Start. In order to accelerate the process, you can start the transportation activity before the completion of manufacturing.
Lag is the delay of the successor activity. The successor activity cannot start right after the end of the predecessor activity. In all relationships Finish-to-Start, Start to Start, Start to Finish and Finish to Finish, lag may be found. Lag time can be considered a positive value.
For example, you are managing reinforced concrete works and you have two activities in sequence which are rebar and formwork activities. Formwork activity duration is 10 days, rebar activity duration is 5 days. You can use Start to Start + 7 Days Lag.
Another example is as follows
After completion of rebar and formwork, you poured concrete. Now you have to wait for 14 days to remove the formwork. Because concrete strengt must be adequate for this process. The relationship between concrete pouring and formwork activity should be Finish to Start + 14 Days Lag.
Lead and Lag time are two basic concepts used in the preparation of a project schedule. Project lead time can be considered as a negative value and can be used only with Finish-to-Start relationship. Project lag time can be considered as a positive value and can be used with all the relationships. Both of them are used to represent the real work which will be performed.