Originally posted by Ringtailhunter
To be honest I have never used fritz. I have read quite a bit about it though. Shredder will look a little further ahead.
There are two things that improve strength of engines; search depth and complexity of position analysis.
From what I have read Shredder uses quite aggressive pruning (will discard unlikely looking lines so as not to waste time analysing them) and uses a slightly simpler position algorithm (the formula it uses to see if a position is good or bad) to make gains in search depth.
Hiarcs is the opposite, it doesn't prune as much and uses a more complex algorithm which slows it down but means it will determine the goodness of a position more accurately.
Fritz is in between the two.
Now obviously with competing ideas like this there is likely to be an optimum point. Of course the problem is finding it.
Attempting to search deeper at the expense of algorithm complexity has the problem that as you move through the ply (half-moves) the number of positions to check grows at an alarming rate (even with pruning) and the position scores could be a little shakey (see the game known as the Alterman Wall for an example of a position algorithm failing [the game also contains a out of horizon plan]).
Too heavy pruning means that sometimes the engine will completely disgard a line that could be winning but not for say 13 ply (well within search range but anything involving say a queen sac on ply 2 will likely have been pruned) until the secondary search (a more complete search that follows behind the primary search) picks it up. Shredder seems to suffer from this.
Attempting to check every position with no pruning (Deep Blue was the last engine to do this) or use an extremely convoluted algorithm means that the seeing horizon of the engine is reduced and any plan exceeding that horizon will remain unseen perhaps until it's too late.