In Primavera we can define Critial activities by 2 option:
There are many reasons why we should choose Longest Path instead of Total Float. One of the reason is that when our project has multiple calendar.
We have 4 activities which have same calendar : Standard 5 day workweek. We use Total Float to calculate Critical path. The result is this:
When I change A1020 activity’s calendar to 1 Day Workweek. We use Total Float to calculate Critical path. The result is this:
You can see it’s wrong critical path.
Now when you choose Longest path option. It will become like this:
Now the critical path is better π
So when you have multiple calendar project, consider to use Longest path option.
Khuong Do Blog 
This is what I like to call “An Undocumented Feature”. translation; a Primavera Gotchya that will sneak up and getchya when you least expect it. This explains why I have seen some really odd critical paths on my project.
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how would you apply this if you’re using MS project since there’s no “Longest Path” option in it?
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That’s why I think P6 is more professional than MS Project. You should change to P6 π
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