Force

Force is a combat mechanic in Dragon Age II that reflects a secondary outcome of dealing and receiving damage. When a physical or magical attack connects with its target, the "force" of the attack determines the extent to which the target is delayed in carrying their intended action, ranging from no effect at all to being knocked clear to the ground. 
Force is calculated as:
Attack Force = (Attack Damage * Force Multiplier) - Fortitude
The amount of damage done (after taking into account armor/resistance and damage reduction) is multiplied by the force multiplier of the attack, e.g. 10dmg*x3-multiplier = 30 initial force. Fortitude (force resistance) is subtracted from the initial amount of force that is received, e.g. 10 fortitude = 30-10 = 20 force taken.
The amount of force taken relative to the target's maximum health one confers one of the following recoil effects:
# <10% of max health = No interruption, target carries out action as intended
# <20% of max health = Slight Knockback: Target is knocked back a step with a brief interruption in action 
# <30% of max health = Full Knockback: Target stumbles back several steps and is unable to carry out an intended action for a longer period of time than slight knockback
# >30% of max health = Knockdown: Target is swept off their feet and crashes to the ground, unable to take any action until they get back to their feet, a process that takes several seconds
Recoil effects can be easily renewed by the attacker continuously using the original effect frequently enough that the target cannot recover in time to dodge it. Examples include an Ogre's ramming attack and an enemy two-handed warrior's basic strikes. Such situations can spell certain doom for an isolated target.
There are several methods available to reduce the frequency and severity of these recoil effects:
* Increase armor rating/damage resistance = less damage taken
* Increasing Fortitude, through equipment or investment in Strength = less force taken
* Increasing maximum health, through equipment or investment in Constitution = less force taken relative to max health
* Passive abilities and equipment with immunity to knockback/knockdown properties (Note: conditions are mutually-exclusive; immunity to knockback does not automatically confer immunity to knockdown and vice-a-versa)
* Not getting hit via dodging (via moving away or defense) or not being targeted (threat management and crowd control)
Additionally, elemental forces from attacks such as Fireball or Bursting Arrow can add additional conditions on top of being interrupted and knocked around, namely immolation from said fire attacks. These can be mitigated with magic resistance as well as resistance tailored specifically to the element of the attack in question.
After some experience and gaining greater crowd control abilities it is possible to get by quite easily via the methods listed above. However, for the initial levels while constitution, fortitude, and armor/damage resistance are low, interruption and knockback, even knockdown, will remain issues to be addressed.
---Defense---
In more mathematical terms, to cause knockdown near the start of the game, with 0 Fortitude, enemies must inflict 30 damage on a mage (default health 100), 37.5 on a rogue (125), and 45 on a warrior (150) in a single attack, or 15, 18.75, and 27.5 respectively with a 2x multiplier. This of course does not take into consideration the aforementioned methods, which will increase naturally over the course of the game, particularly for warriors.
As enemy attacks become more initially forceful i.e. have higher damage and force multipliers, before Fortitude's reduction kicks in, a 5-point increase in health from adding an attribute point to Constitution will do more to reduce the final outcome of the attack than a 1-point increase in Fortitude from adding that point to Strength. Under the right circumstances, a maximum health as low as 105 can cut the final outcome by a marginally better magnitude than 1 Fortitude: 
:At around 1-18 initial force, it is better to deduct the initial force value with Fortitude
:At 19-23 initial force, the player has a roughly even outcome in favoring Fortitude, health, or some combination of the two
:At 24+ initial force it is more advantageous to divide that value through a larger health pool than trying to deduct it with Fortitude
Effectively, each point in Constitution raises the amount of damage an enemy must inflict with that same attack by 0.5 to cause slight knockback, 1 to cause full knockback, and 1.5 to cause knockdown. At least where full knockback and knockdown are concerned, this can generate a better return than Fortitude.
These figures in turn help illustrate why rogues, with a default health of 125, are inherently more resilient than mages, and warriors, with a default health of 150, are even more resilient than that. Some powerful attacks intended to cause knockdown e.g. around 40 initial force (a threshold of 30 is easily mitigated by a warrior's default health value, anything much higher will likely either carry very high base damage (like an assassin's Backstab) and simply KOs the average mage or rogue, making force a moot point, or has such a high force multiplier that it would knock down anyone but with relatively little damage) can be mitigated to a mere knockback with around 35 additional health, either through accessories or investment of a couple level-ups in Constitution.
However, reducing such attacks to the point that no knockback occurs (below 10%) whatsoever will require an even greater spiking of maximum health, i.e. roughly 20+ more points in Constitution, which are better spent in the class's primary attack stat due to the game's combat mechanics. This in turn retains the value of passives like Resilience and Unshakable, as well as accessories that confer "Immunity to knockback." Furthermore, warriors' heavy armor have higher armor ratings compared to the medium and light available to rogues and mages respectively, so they will always incur lower initial force. With these points considered, Strength remains the most critical stat for warriors, and Constitution may not be suitable for rogues and mages in lieu of raising their offensive attributes and simply avoiding getting hit.
To avoid knockdown, final force must be reduced to <30% of a player's maximum health. At an initial force of 100, it would take roughly 335 maximum health, so only outright immunity (e.g. Aveline's Indomitable talent) or dumping every point from level ups into Constitution will prevent it. At around 70, it takes up to 235 health. At 50, 170 health. At <30 initial force it's not possible for the attack to even cause knockdown. When character health gets into the 250s it pushes higher initial force amounts present in the game back down to a threshold of 23% of maximum health, at which point the player will better reduce the force even further with flat rate Fortitude.
To summarize, Fortitude is better for reducing knockback to mere interruption, and then shrugging off interruptions if the force can be reduced enough. Taking knockdown abilities down to knockback is more easily accomplished with investments in maximum health rather than Fortitude.
---Offense---
As for going on the offensive, characters can inflict a much greater amount of damage with their talents, provided they upgrade their weapons regularly and take passives to increase damage. On Nightmare difficulty, Critter-ranked enemies have ≈100 health, Normal ranks have ≈500, and Elites have ≈1000. As early as level 4 most characters but especially Bethany and Varric can inflict sizable chunks of damage with talents that also carry 2 and 3x multipliers, effectively giving them "initial forces" of 200-400. Whatever Fortitude the enemy has is often insufficient as these basic, early-acquired attacks sweep many of them off their feet, immolate them, and all the while interrupt their own attacks. Shield Bash has a staggering 12x multiplier and is available at Level 2. With a strong enough one-handed weapon in hand can easily cause knockdown on a swath of enemies even with relatively low damage. Talents and spells that combine a base damage value with moderately high force multipliers are great additions to any character, especially if these properties can be upgraded in lieu of spending that talent point on abilities that are more or less redundant, less effective, and require a greater number of talent points just to unlock. Namely, cross-class combos will further improve abilities by adding even more damage and force for fulfilling their setup conditions, but will often require upgrades to existing activated talents and spells rather than new ones.
Category:Dragon Age II gameplay