What algorithm does MS Project use when calculating BCWP

Specifically I'm interested where the Earned Value Method is set to Percent Complete.

I have been having a play around with a couple of simple short duration tasks.

• Each is 10 days where the work is front loaded before being baselined.
• I set the status date half way through.
• On the first task I recorded progress pretty much to plan.
• On the second task I delayed the start by 4 days then in the one day left of the week I only recorded actual work equivalent to half that was planned on the first day. I also extended the duration of this task to 20 days.

When looking at the Time-Phased EV fields in a Task Usage view on the first task I see what I expect to see.

On the second task I see Cumulative Percent Complete as 5% (correct for one day in 20). If it had followed baseline duration then it would be 10%  rather than 5%.

However the BCWP for the task is a bit more than the half the BCWS for the first day of the task. Given that the actual work was half of the baseline work, I would expect BCWP to be half of BCWS.

For Earned Value Method = Percent Complete, MS Project obviously takes into account:

• Baseline Duration
• Time-Phased Baseline costs
• Current Duration
• Cumulative Percent Complete

There may be more that I have not considered but is there any documentation I can reference that describes the algorithm that Project uses in calculating BCWP so I can get a better understanding of what to expect.

Thanks