Calculating Schedules

Calculating schedules in LeanCPM is slightly different than in legacy CPM.

Main Differences from legacy CPM

1. There is no concept of constraints (As soon as possible, Start no earlier than, etc.)
2. There is no data date, only Today
3. Every milestone has a target date, used to calculate the earliest and latest dates by which all tasks can occur.
4. Nominal dates (default Start & Finish) use manually entered ‘Target’ dates instead of early dates.
5. There are only two types of dependencies (relationships)
   • Series: A blocker must finish before the blocked task can start; lag is not allowed
   • Parallel: A start-start and finish-finish relationship with an optional lag
6. Milestones can only be in series with their dependents and dependencies
7. Schedules are calculated in days, not minutes or seconds.
8. Issues are a mix of milestones and tasks

What exactly is calculated?

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What is ignored

Anything that has a “canceled” status is not considered in the calculation.

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Needs update

Since there is no data date and therefore no “progress spotlight”, we need a way to identify tasks whose progress may be stale and need to be updated. Fortunately, this is easy.

First, any unstarted task with a target start before today needs an update.

Second, any unfinished task with a target finish date that is before the current date needs an update.

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What is required

Earliest dates

The earliest a task can start is right after the earliest its driving blocker can finish (or start if it’s in parallel).

To calculate the earliest a task can start, we must examine a given task’s blocker, then its blocker, and so on, until we eventually reach a task without a blocker. Since tasks in LeanCPM don’t have constraints, it’s impossible to determine the earliest a task can occur without a milestone or an issue that initiates the dependency path.

What about tasks that have cleared blockers?

These are considered Ready to Start and have an early start as of the current date or the Target Date, whichever is earlier (see 'Needs Update' above).

What about tasks with no blockers at all?

Unless they are already started or their Target Start is the current date, it is impossible to figure out the early start.

Latest dates

All tasks must have dependent tasks that eventually lead to a Milestone; otherwise, it'll be impossible to figure out the latest they can start or finish.

Therefore, in a given dependency path, it’ll be impossible to calculate the total float, drag, and priority of the tasks that are not between a cleared blocker, milestone, or issue.

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Forwards Pass - Calculate Early Dates

Early dates are the earliest a task can start.

Step 1: Unstarted Tasks with no blockers

A. Target Start is after Today

We can’t assume these can start earlier than what the user set as expected.

1. Clear Early Dates

B. Target Start is before Today, Finish is after

1. Set early_finish = target_finish (user-generated)

C. Milestones before Today

1. Do nothing

Step 2: Blocked Tasks & Milestones from previous step

Now that we’ve identified the early dates for a handful of tasks, we can begin calculating early dates for the tasks they’re blocking.

A. For each in series

These tasks can’t start until the tasks blocking them are finished.

1. Set task’s early_start to blocker’s early_finish + 1d (work day)
2. Set eary_finish = early_start + duration
3. if the task's early_start or early_finish is blank

B. For each in parallel

These tasks can’t start until the tasks blocking them also start; sometimes, there is a lag. Since the blocked task can have a shorter duration than its blocker, its early dates are calculated a bit differently.

1. Set task’s early_start to blocker’s early_start + lag
2. Set task’s early_finish to the later of:
   • Blocker’s early_finish (do not add the lag) OR
   • Task's early_start + duration
3. Milestones cannot be in parallel with other Tasks or Milestones

Step 3: Repeat Step 2 until there are no more blocked tasks

Step 4: Constraints

Iterate through each constraint bucket, find any conflicts. If a conflict exists

Step 5: Move onto Backwards Pass

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Backwards Pass - Calculate Late dates

Late dates are the latest a task can occur

1. Set late dates to null
2. Find all tasks without a dependent (successor) a. for each: set priority to unknown set late_finish to finish (user-generated) set late_start = late_finish - duration

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Free Float calculations

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Total Float calculations

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Drag Calculations