This is an unofficial explanation of how item power is calculated. It has been created due to ongoing confusion about minimum rolls vs item power. The information is current as of July 2021 and has been since Title Update 10.1.
From the TU10.1 patch notes:
Increased minimum item power and chances for higher power items for several difficulties, resulting in higher average rolls overall
Status Effects = 100%Skill Haste = 66% (8/12)
On SOTG #165, this graph was presented to illustrate the loot power distribution for heroic content - note the 60% minimum average roll:
Why are the bars the "wrong" size?
In the example above, the status effect roll is 6.6% vs a maximum of 10%. Or 66% of the maximum. Yet the bar is less than half? 😕
This is because the bars don't start at zero, but at the minimum roll for a given attribute. 💡
If the minimum roll is 5% at L40, then the bar can show a maximum range of 5% (10% maximum - 5% minimum). To calculate how big the bar should be: 1.6/5 = 0.32 or 32%
What about Skill Tier?
Items that roll skill tier as the core attribute ignore that when calculating item power (as it is always maxed).
Item Power = 83% = (100 + 66) / 2
TU10.1 patch notes: https://forums.ubisoft.com/showthread.php/2249829-Title-Update-10-1-Patch-Notes-Updated-11-00-AM-CEST-July-20
State of the Game #165:
State of the Game #168:
Gear 2.0 attributes (for minimum/maximum rolls):
Nikki later clarified that the 60% average for heroic content was an example, but has never revealed the actual %