Diff between BCWP and ACWP

[Joe]

I know I am missing something here, I am hoping someone
can explain in simple terms.

In general terms and at a given point in a project, if the
BCWP is less than the BCWS, then I understand that the
project may have been underestimated because we are now
behind schedule. What I am struggling with is the ACWP,
wouldn't this be the same as the BCWP? For example, If we
planned to work 40 hours by today at $1.00 an hour, but we
only worked 32 hours (or $32), then wouldn't the BCWP and
ACWP both be $32? How and when would ACWP and BCWP be
different? I know they can and will be, I just don't
fully understand how and why? One thing I am thinking is
if someone works overtime to make up for lost time that
would drive up the ACWP, but maybe not necessarily the
BCWP. Can some explain a simple scenario in plain English?

Thanks!

 

[Steve House]

As you say, generally speaking the ACWP and BCWP will be equal EXCEPT
when something has happened to change the cost of a resource. Let's say
you have a 10 hour task with a guy assigned to it at $10 per hour. He
gets sick so at the last minute you have to substitute another guy who
happens to get $12 per hour. When it's completed the BCWP is $100 but
the ACWP is $120. Other things that can cause a variance between BCWP
and ACWP are things like materials costs - you estimated it would
required 100 cubic yards of concrete at $2.50 each but actually used
only 90 cubic yards - and fixed costs - airfare to the job site was
estimated at $500 in the original budget but when you booked the flights
it was really $750.