>>15714991Here's how it works.
A mon's ability to take damage is essentially multiplicative.
A mon's ability to take a special attack is essentially determined by multiplying its SpDef and HP, and the higher that number the more survivability it has.
Therefore, the best bet is to try to get (SpD*HP) as high as possible.
Strictly, that means making the final stats even. So high HP and low special defense usually calls for investing more in special defense, whereas high special defense and low HP calls for investing more in HP. This is because maximizing area involves making a square; 9*9>10*8>11*7>etc... Get it?
Now here's where it gets tricky. HP is, by default, almost ALWAYS a better investment for bulk. This is because HP affects both defense AND special defense. To return to our 9*9 example, say our base stats are 9HP, 7 Def, and 7SpDef, and we can assign 2 points. 11*7 is 77, rather than 81 for investing those points in a defense stat... But the average of 77 and 77 (11*7 for both stats) is 77, while the average of 9*9 (81) and 9*7 (49) is 65.
Make sense? HP EVs essentially count "twice" since they affect both bulks.
Things get trickier with multiplicative items and abilities; if you have an Assault Vest, your investment changes further, because any investment in SpDef automatically gets bumped by 50%; so it becomes a question of investing 2 points in HP, or essentially 3 points in SpDef. Natures work the same; because they can boost a defensive stat by 10%, you get a free stat point for every 10 stat points you invest.
And then there's the question of intimidate, which lowers your opponent's attack automatically and so initially buffs your ability to take physical hits, meaning they require less investment.
