Wasn't going to post this in specific forums as it applies to all tanks.
Based on this
Dev comment Bosses have 90% accuracy on their basic M/R attack, and 100% accuracy on special M/R and F/T attacks. This is the same as player base. Accuracy on certain attacks can be increased to unavoidable levels if it is required by a mechanic.
SWTOR uses 2 roll system for determining whether and how hard attacks hit. You can read about this on mmo-mechanics and tor forums. For the purposes of this thread we can ignore the second (Crit v Shield) roll and focus on the Accuracy v Defence.
The first roll is:
IF roll < (accuracy - defense) THEN attack succeeds
IF roll > (accuracy - defense) THEN attack is dodged
Take an attack with 100% accuracy thrown at a tank with 25% defense chance. Then (accuracy - defense) = 75. You would assume 25% defense chance means you dodge 25% of the time, so a dodge would have to be if the roll was higher than 75.
A dodged attack obviously cannot be shielded and provides benefits to Guardians and Shadows. Vanguards prefer to shield instead due to talents but dodging an attack is still beneficial to the raid as a whole (less healing needed).
Tank classes have the following ways of reducing boss accuracy.
Guardian: 5% acc decrease all the time (Dust Storm from Defence tree via the use of Sweep, when used off cd and there is no reason no to, it is free for Hybrids and tends to be free in Defence as well from dodging. It is an AoE attack and has high threat when speeced into Single Saber Mastery and all tanks are. Cyclone Slash also applies this debuff but it is used a lot less.)
Vanguard: 20% MELEE AND RANGED acc decrease for 18s every 60s (From Smoke Grenade, activated ability in the Shield Specialist tree, more spikey but can kinda be thought of as 6% all the time. With the Set bonus it is 6.666% due to 20s duration)
Shadow: 5% acc decrease all the time (Via the use of Force Breach when in Combat Technique, tanking stance, when used off cd and there is no reason not to, it is an AoE attack and has high threat)
I tend to use the Vaguard ability more as a def cd but it is useful at any rate, also should be used by offtank to help out. Shame it doesn't decrease F/T accuracy, I always feel so stupid when I use it on Soa or TFB.
I presume that the Guardian debuff decreases F/T accuracy because Amorphic once reported missing with Sweep after being Smashed by a tank Jugg. Can only guess that Shadow works the same way as there is no indication otherwise.
Summary:
Guardians and Shadows have a passive 5% Defence increase to M/R attacks. Vanguards have an ability increasing their Defence by 20% for 18s/20s to M/R attacks, a theoretical increase of 6%/6.666% over the course of the fight. It is likely that Guardians and Shadows also have a passive 5% resistance increase to F/T attacks but unknown.
Subject: Accuracy on Operations Bosses. 
Wed Nov 21, 2012 1:23 pm
