"If the painter does that but paints your house purple instead of white, then you wouldn't have a claim against him," she says.

Scafidi has taken such advice to heart. Instead of saying a task will take "a reasonable amount of time", Scafidi writes: "The specified task will take no more than four hours".

"Attorneys feel good when we have a clear, unambiguous definition on things like that," he says.

Remember postproduction needs. Guerrero recommends including postproduction requirements in the SOW. Spell out the testing and support you'll need from the vendor, she says. And if you plan to have internal folks support the system after installation, the SOW should address whether the vendor will train your staff. Such language, she says, guarantees that the vendor doesn't "just deliver the system and walk away".

SIDEBAR: An Earned Value Management Project Primer

Earned value management is based on several figures that are used to calculate a project's progress.

You can measure in dollars or time

Planned value (PV): This is the value of all resources needed to do the work to meet the project's objective. Although most project managers calculate PV in dollar terms, some calculate it in terms of time - the number of hours it's expected to take to complete the project.

Let's take a very basic example. We've budgeted $200 to buy, set up, network and test a new system. We've budgeted $50, $75, $50 and $25, respectively, in materials, labour and other costs for those four phases.

Keep in mind, though, that the $50 set aside to buy the system doesn't just cover the cost of the actual hardware and software. It also takes into account the value of time that will be required to find the right system, the time that will be needed to fill out the purchase orders, the time it will take to actually buy the system and so on.

"The basis for earned value management is work performed, not money spent," says Marilyn McCauley, owner of McManagement Group, an EVM consulting and training firm. Our PVs are $50, $75, $50 and $25.

Budgeted (cost) at completion (BAC): This is the sum of all PVs - the total for all phases. In our example, BAC is $200.

Earned value (EV): As our team completes portions of the planned work, we check off that work and the amount of money (or time) it should have taken to do it according to the project plan. Project managers calculate EV at predetermined times based on the plan, typically at the end of the company's accounting period, McCauley says. We've completed Phase 1 - buying the system - within the planned time frame. Check that off as done. Our EV is $50.

Actual cost (AC): This can also be measured in dollars or time. In a perfectly executed project, EV and AC are the same. But in our example, let's say we actually used $60 in resources to buy that system. Our AC is $60.

Once you have these figures - PV, BAC, EV and AC - you can calculate other numbers that tell you about your progress on a project. Here are some of those calculations:

Schedule performance index (SPI): EV divided by PV for a particular phase of a project. In our example, that's 50/50 = 1, a perfect score for Phase 1, indicating that we're on target for schedule. "I said I'd do $50 worth of work, and I did $50 worth of work," McCauley says.

Cost performance index (CPI): EV divided by AC. For our project that's 50/60 = 0.83, indicating that we're underperforming for our costs. "For every dollar I'm spending, I'm only getting 83 cents worth of work," McCauley explains.

In a perfect project, the answer is 1. But most projects fall below that because most projects miss their targets.

Estimated (costs) at completion (EAC): BAC divided by CPI. The answer is a forecast value in either dollars or hours that indicates the projected final project costs or time. There are various formulas for EAC, McCauley says, but this is one of the easiest to use. In our example, that's 200/0.83 = 240.96. This indicates that at the rate we're going, the final cost will be $240.96 rather than our planned $200.

Schedule variance (SV): Subtract PV from EV. In our example, our earned value is $50 because we've done the first of our four phases: We bought the system. The PV for that first phase was actually $50. So 50 - 50 = 0. That's a perfect score, so we're on schedule.

Cost variance (CV): Subtract AC from EV. In our example, that's 50 - 60 = -10, indicating that we've overspent by $10. If we were on target, CV would be zero.