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Critical Activity Definition

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User offline. Last seen 2 years 8 weeks ago. Offline
Joined: 18 Jul 2014
Posts: 13


Hi All,

Somebody gave me a Construction Activity schedule but when I run it using "Total Float less than or equal to Zero (0)"  I saw no critical activity but when I changed to "Longest Path" then I saw my critical Activities in Red Colour.

Can anyone explain the best way to define critical activity and what is their pros and cons between these two?

1. Total Float less than or equal to Zero (0)    or

2. Longest Path




Gary Whitehead
User offline. Last seen 2 years 34 weeks ago. Offline

In a schedule without hard constraints or out of sequence working (ie a good schedule), it will make no difference.

If you do have hard constraints and/or out of sequence working then:



1) Helpful to highlight out of sequence issues to enable you to fix them

2) Gives you a filter showing all activities which will need to be brought forward or maintained in order to achieve your hard constraints and/or completion date


1) Neither of the above are necessarily the critical path -fixing an out of sequence issue or accelerating an activity with negative total float may or may not bring your completion date forward, depending on the rest of the programme.

2) If you have hard constraints in the middle of the project fr which predecessors are running late, these will show as critical despite having no impact on your project end date.

3) If you have a hard constraned project completion date, and are currently forecasting earleir completion than this, the filter will not show you the critical path, since it has positive total float.

Longest path:


1) Shows you the  actual critical path through the project -i.e. the activities which if delayed will delay the project

2) Shows only the critical path, so you don't get confused by what is a truly critical activity, and what has negative float for other reasons


1) won't show you all activities that have to be brought forward to achieve a targeted (hard constrained) end date, only those that are currently defining the planned end date

So it largely depends on what you want to use the filter for. If you want to use it to understand why your planned end date is where it is, use lnogest path. If you want to use it to find problem activities that need resolution, use TF <=0.