Lead is the process of acceleration of conducting successor activity by starting the successor activity before the finish of previous activity, this can be done if there are no physical or technical constraints on starting with successor activity before completion the precedent activity, this will occur in FS type of activities.
An example of this is the fixing of formwork and casting of 50 columns for a building, fixing formwork required 4 days. instead of waiting for 4 days, to begin with, casting, we will start casting from day one. skilled engineers will always utilize all the available manpower and equipment. also, they will reduce the time of performing the job by overlapping the activities as much as possible without reducing the quality of the work.
Lag is the delay of successor activity after the completion of the precedent activity. lag represents the time before the second activity can start. Lag may be found in activities with all relationship types: finish-to-start, start-to-start, finish-to-finish, and start-to-finish.. suppose we finish casting our bridge deck and post-tensioning is the second activity, post-tensioning can’t start until the concrete attains the required strength so in this case second activity delayed after the completion of the first activity and this is the lag.