Dota-Allstars Attack Speed Guide will be posted here. This guide is made by Malle from www.dota-allstars.com , all the credit belong to him.
( 1 + IAS ) / BAT = Attacks Per Game Second
BAT / ( 1 + IAS ) = Game Seconds Per Attack
BAT is Base Attack Time:
The time in seconds between attacks without any increased attack speed
IAS is Increased Attack Speed:
How much faster you attack (100% IAS = 1.00 IAS)
Each point of agility gives you 0.01 (or 1%) IAS.
IAS stacks additively
Gloves of Haste (15% IAS) and Hyperstone (55% IAS) will give you a total of 0.7 (55%+15%=70%) IAS.
Negative values (from something that slows you) is just removed from your IAS
Take the above example of 0.7 (70%) IAS but you get slowed by Untouchable lvl 3 for -0.90 (-90%) IAS, you end up at -0.2 (-20%) IAS.
The final IAS affecting your attack can range from -0.8 to 4. Anything less than -0.8 will act as -0.8, and anything more than 4 will act as 4.
Maximum attack speed is thus:
5 / BAT = Attacks Per Game Second
BAT / 5 = Game Seconds Per Attack
And minimum attack speed is thus:
0.2 / BAT = Attack Per Game Second
BAT / 0.2 = Game Second Per Attack
To see which is more effective to get, IAS or damage, you can use the following formula:
X = Y * ( 1 + IAS ) / DPA
Y = ( X * DPA ) / ( 1 + IAS )
DPA = Damage Point Average (the average damage per attack)
IAS = Increased Attack Speed
X = amount of IAS needed to equal Y damage
Y = amount of damage needed to equal X IAS