Question About Pooled Mining

[LGT06]

I understand the general theory of pooling resources and splitting the rewards based on work contributed, and I've read the pooled mining wiki page at https://en.bitcoin.it/wiki/Pooled_mining, but I have one question about a technical detail of pooled mining.

When someone contributes a share, does that affect the work the rest of the pool does? Does the share just show that work was done therefore the user is entitled to some reward, or does the share show exactly what work was done so other miners in the pool avoid using the same nonce values that don't solve the block, for instance?

Does a miner who has contributed shares but not solved a block help the other miners in the pool to solve the block more quickly, or is that miner getting paid for doing work that hasn't actually contributed anything valuable?

[os2sam]

I'm pretty sure all miners are given unique work items.

[Trongersoll]

The way i understand it, is that all the miners are working on a unique starting point and running through all the nonces for that attempt. it really is a race between all the miners, not each building on the work of the others. also, once a block is found regardless of if it is in pool or out, everyone has to stop what they are doing and start on the new block.

[notme]

Right, if it takes 1 terrahash to find the right nonce, as far as the bitcoin network is concerned, 1 terrahash - 1 hash of that work can be thrown away.  However, the pools track a lower difficulty "share" and use those shares to calculate a payout when a pool member finds a block.