The forward pass process determines the following.
The earliest dates from the start and
the finish of each construction activity you do have
in your project, and this is one.
The second also determines the minimum duration of your construction projects.
This is the two main things that we generate and calculate from the CPM or
the Forward Pass Procedure in the scheduling technique here.
The backward pass process, on the other hand,
determines the latest dates from the latest to start and
the latest finish of each construction activity or
task that can be performed but
without increasing the construction projects minimum duration
that had been identified in the first Forward Pass calculation,
or the first step of the CPM scheduling procedure calculations.
So, lets go through an example here, from the Forward Pass calculation.
If you do remember, we covered in another module what we
call the key for your diagram.
In this aspect, we have the key here
which is showing in the square the activity name.
Let's say A, B, or C,
concrete pouring erecting steels, or taking the forms and so on.
And below it we will have the duration in days or in weeks or in months.
So as we can see here, the early start date of activity A is 0.
And once we given the duration of the activity
in this example we have one day.
In this case, we will have the early finish date of activity A is 1, and
the equation, if you want to look from a mathematical point of view,
the early finish of the activity in the forward pass
calculation is equal to the early start of that activity which is,
in this case, we have zero plus the duration,
in this case we have one day for activity A which will give us one.
I always recommend to everyone not to memorize these mathematical equations.
I highly recommend for you to understand the flow of how you build the Forward Pass
calculations, even though you have a lot of softwares out there,
like Microsoft Project, like Primavera P6 that can do the calculations for you, but
it is very important to understand the concepts and the foundations behind that,
so as to be able to read it, to be able to understand where it came from.
So once we understand then, what is the early start date for
activity C and D, we will then have to look
at the duration of activity C and activity D and then from that,
we will find the early finish of both activities, which in this case for
activity C the early finish will be the early start which is 1+6,
which will equal to 7, and the same go for activity D.
If we have duration is 13, then the early finish for
activity D will be early start of 1 plus the duration of 13,
which will all well to be 14 days, the early finish of Activity D.
So in this example, what we highlighting is that relationship between
the activities of finish to start which is the traditional
relationship that I explained in the previous modules, that I highlighted,
that most of schedulers we have, they do prefer to follow the traditional
module here, the traditional relationship, I'm sorry, in this example.
So that will held correctly if we have this specific relationship.
Also, another point I want to highlight in the forward pass calculation
is what if we do have not just one activity
like A that C is the successor for?
So let's say for example, we have another activity here called activity B.
And also it has a relationship between B and
C, which is finish to start.
In this case, the early start date for activity C will be
the maximum number between all the early finish dates
of the activities that predecessor or before that activity.
The first activity again is activity A, so we'll start by the assumption,
we'll start by zero.
So, the early start date for A is 0 and
the early finish date of activity A is 1 which is 0 plus 1 equal 1.
In this case the successors activities of activity A will be B, C and
D and the early start for each of these activities will be the same as
the early finish of the predecessor A which would be one.
So, early start for B one, early start for
C one and early finish early start for D is one.
that will not satisfied the relationship that we have in our project here.
Because if the early start date of E equals 7,
that's mean the activity B will intersect of its relation with E.
So, I have to wait until the relationship of both are correct
and finish both activities which will have the date of
10 to then start with activity E.
So in this case, activity C even if we finish it at day number 7,
we still have three days, kind of a buffer, to wait until
let's try to finalize the end of the project,
the last activity in the project which is activity G.
The early start date for that activity will be
the maximum number between all the predecessors
activities their early finish date.
Which in this aspect would be 17 because we waiting for
activity E and F and G to finish all three
to start with activity G, so the last one will finish is activity E,
then the early start date for activity G will be 17,
then the early finish date for
activity G will be 18.
So, if you remember in the beginning of the introduction of this module,
I highlighted that the forward past calculations, it will calculate or
determine the earliest time and the earliest
time we start an activity, and the earliest time we finish an activity
without kind of effecting the duration of the project.
Second, is we highlight that minimum duration of the construction project.
So in this case, what do you think the minimum duration of the project here?
In this case, it will be that early finish date
of the last activity we have, which is 18.
So the duration of our project would be 18 days.
So these are the two aspects we determine
when we look at the forward task calculations.
Calculating and Executing a Forward and Backward Pass
When constructing a project schedule, a project manager can draw upon a score of strategic lessons from the game of football. For example, in wearing the hat of team coach, the project manager has at his fingertips a dynamic playbook of offensive and defensive moves to keep the project on schedule. During the early stages of the game, the project manager's focus is on developing realistic estimates for the project's schedule. The estimation process is accomplished by pulling from one of the early chapters of the playbook two essential plays: the forward pass and the backward pass. Analogous to the two minute drill, the forward pass is the best strategy for quickly driving the ball to the goal line or in the case of a project to the completion date. Each task represents a first down marker and the forward pass pushes the ball along the schedule in the least amount of time. A forward pass is executed by calculating the early start and the early finish for each task of the project. The early start is the earliest point in time when a task can be started. The early finish is the earliest point in time that a task could be finished. However, sometimes the forward pass is not the best strategy, particularly when the defense rushes the quarterback. In this case, the project manager needs to execute a backward pass to see if there is enough slack time to allow for a different set of plays and still get the project to the goal line before time expires on the clock. In constructing a backward pass, the project manager reverses the process to determine the late start and the late finish for each task in the project and remain in regulation time. The late start date is the latest point in time when a task can be started. The late finish is the latest point in time that task could be finished.