What does "forward & backward" calculation mean?

The Critical Path Methodology includes that a task schedule should always be calculated forwards and backwards.

While calculating forwards the first task of each available path starts on a user-defined start date while all successor tasks are scheduled based on their dependencies to the first one and their durations.

The backwards calculation works in a similar fashion using the user-defined end date of the last task of each path, to then schedule all predecessor tasks according to their dependencies and durations.

The results of both calculations are afterwards compared, leading to numerous insights ranging from the Total Float of all tasks to being able to define which path is in fact the Critical Path of the programme.

 

How does it work on Sablono? What are the outcomes?

The Sablono scheduling engine works based on the Critical Path Methodology. The following paragraphs illustrate which user-defined dates are used in the forward or backward calculation, which dates and KPIs are calculated and how to interpret those.

Forward calculation

For Sablono to be able to calculate the schedule of your Activities forward, a user-defined start date needs to be available for the first Deliverable of the path you are currently looking at. If that is defined, Sablono will schedule all Activities of the path according to their (successor-) dependencies and durations. The forward calculation will result in earliest start and end dates for all Activities on the path. The earliest start of the first Activity and the earliest end of the last Activity of each Deliverable will also define earliest start and end for the Deliverable.

Note: Only the user-defined start date of the first Deliverable of the path influences your schedule. Following Deliverables, which are connected to the first one with a dependency making them a successor will be scheduled based on the dependencies and durations of your activities which might conflict with the user-defined start dates.

Backward calculation

Sablono will calculate the schedule of your Activities backwards based on the user-defined end date of the last deliverable of the path that you are currently looking at. If that is available, the path including all Activities on it will be scheduled based on the (predecessor-) dependencies and durations, resulting in latest start and end dates. Once again, the latest start of the first Activity and the latest end of the last Activity of each Deliverable define its latest start and end date.

Note: Just like the forward calculation, the backward calculation will also ignore user-defined end dates for all Deliverables which are predecessors of the last Deliverable of the path.

Total Float calculation and interpretation

The Total Float of Activities and Deliverables shows how much buffer-time is available to finish them between earliest start and latest end. It is calculated by comparing the earliest to the latest start and the earliest to the latest end. The calculated values can be categorized into 4 cases:

  1. Total Float > 0 - Means that you and your project team has a buffer to finish the Activity or Deliverable that you are looking at between its earliest start and latest end.
  2. Total Float = 0 - Means that there is no buffer available to finish the Activity or Deliverable between earliest start and latest end. The Deliverable that you are looking at will most likely be part of the Critical Path of your project schedule. A delay on an Activity of that path will result in a delay of the project. You will also realize that earliest start and end as well as latest start and end show the same date.
  3. Total Float < 0 - Means that you will not be able to finish all Activities between your user-defined start and end dates of the Deliverable that you are looking at. This might happen if e.g. the durations of your activity sequence do not fit to the durations you defined for your deliverables.
  4. Total Float = ??? - Means that Sablono was not able to calculate a buffer for your Deliverable or Activity. This will happen if the platform was not able to calculate either earliest or latest dates while scheduling forwards and backwards. A possible reason might be that you did not set a user-defined start for the first or a user-defined end for the last Deliverable of the graph.

Behind schedule and Forecasted Delivery calculation

Sablono will always evaluate the state of your project based on the latest end dates of your Activities. That means that an Activity is only marked as running behind schedule once the latest end date of it is in the past and it was not finished, yet.

Similar to that, the Forecasted Delivery also compares the latest end date of the Deliverable to the amount of work that still needs to be done to finish it.

Only if Sablono was not able to calculate the latest start and end dates (for the reasons already mentioned earlier), it will use the earliest end to evaluate if your project is still running on time.

The Sablono platform offers a full-blown scheduling engine based on the Critical Path Methodology. Read on, for more information about Sablonos' scheduling modes as well as general scheduling settings.

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