Project Management Metrics

What are the commonly used metircs for managing software development projects through Contractors if the contract is based on Time and Material and is not a fixed price contract.
I have heard of things like schedule variance, first time right, defect density etc. But all this would be good for a fixed price contract. What good be a good metric for contract, where I pay based on the effort put in by the contractor and how do i ensure that the effort is the right effort.
thanks