I believe Critical's are only based on each individual power. Some, for example Broadsword's lvl 32 attack, Headsplitter, have a higher critical chance. AFAIK there's increased chance of one when at low health.
Defiant 50's
Many and varied!
@Miss Chief
Pretty sure its random m8, a lot of time they come fairly regularly sometime you can do a mish with maybe only one or two crits, dunno if it's just me but a lot of the time it always seems to crit on parry 
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AFAIK there's increased chance of one when at low health
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I think you may be thinking of scourge, the corrupter inherant that does indeed happen when the target gets below 20% hp, but scrapper citicals and random based on the power (i think most of the lvl 32 ones have an increased chance to crit).
Acc also has no effect on this, nor does build up, though on CoV (and maybe CoH? i'm not sure) a held target increases the crit chance from a stalker by a further 20%.
And yes it always seems that others get crits all the time against us in PvP, but thats more to do with perception i think...we remember when we are planted by a one hit critical much more vividly than when we do it ourselves 
It's like acc that way, you can hit 9 times out of 10 all night, but you will only remember that string of 6 misses in a row when you really needed them to hit
I miss him in the weeping of the rain;
I want him at the shrinking of the tide;
The old snows melt from every mountain-side,
And last year's leaves are smoke in every lane;
But last year's bitter loving must remain
~Edna St. Vincent Millay
Rank of foe seems to matter, more crits against bosses and less against minions.
(My experience: Head Splitter only crits when I could have used brawl to finish te foe).
and if you critical it'll always be on the least powerful attack you have... critical on cobra strike, ooo 6 damage critical on parry ooo 40 damage
nice when it happens but certainly not something to plan for hehe
@ExtraGonk
BAH!! typo FTW!!
I meant to type There's NO increased chance of one when low on health!
Defiant 50's
Many and varied!
@Miss Chief
This is correct. Criticals are more likely against higher ranking targets. So an AV gets much more criticals from you then a minion does.
I got a critical on every Dark Blast I used on the last Hami raid, Midnight Grasp and Shadow Maul didn't crit once....... damn mitos have friends in dev places I tells you
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Head Splitter+Build up+Crit= One dead stalker not knowing what happened to him
even better when you get it in before their AS lands hehe
@ExtraGonk
Finally got around to motivate myself into making a CoH char past level 10.
The question, critical hits are purely random? Do the chance of critical hits increase below a certain % of hp left on the target, or it is just random at all. Is there any attacks with greater chance of critical hits?
Asking because i don't find that i land critical hits that often. Kind of funny though, as i always curse while playing my MM or Corruptor when the pesky scrapper two shot me! Seemed that they would land critical hits pretty often.