Snares and IMS stacking

How do increased and decreased movement speed stack together? Is it additive or multiplicative?

In other words, let's say I cast Ice Spikes (66% snare) on a HM monster (they get an innate 33% IMS, or so I hear). Will the monster be at 67% of normal player movement rate (which would be additive), or 43% of normal player movement rate (which would be multiplicative)?

Effects can stack. However, the problem is that there is a limit to how much they can stack up to. For example, speed boosts stack, but only up to 33% - unless a single skill increases beyond the limit (ie. Dash). Same for slow. Wiki states the max stack to slow is 50%.

http://wiki.guildwars.com/wiki/Effect_stacking

P.S. the nature of hard mode boosts is not clear. They could be boosts, and work per stacking rules, or they could be a new base value.

from wiki:
Percentage effects stack multiplicatively, while non-percentile effects stack additively.

That's what I was looking for. Thank you.

I know about the caps already. I was wondering specifically about how snares and IMS affect one another.

Ultimately, it doesn't matter if the monster's speed is considered an IMS or not. Either way, it's getting reduced by 66%. Which means snaring a HM monster is almost as effective as snaring a NM one (at slowing them down, anyway).

strangley enough i do know 2 skills that stack relatively well and are a bitch to get by

Iron Mist AND Muddy Terrain, having monks use shouts suck as make haste negate the speed buffs making it permanent 90% ...and get this they walk in place and really dont move at all...(juggers and turtles) maybe even 100% slower