Disagree with logic on NCEES Problem 505

[petobe]

Problem asks specifically for present worth of the COSTS - why would they include the residual value? The residual value is not a "cost", and it is not realized until the end of the design life. If an agency were trying to determine the amount of money that they needed to put away to cover all costs associated with a 20 year life (basically what the question is asking)... I would argue that the residual value should not be included in this analysis... if it were, the agency would run out of money prior to the 20 year term expiring.

In similar terms - if you buy a car that has $x cost in maintenance every year for 10 years... the fact that you may sell it for $y or $2y at the end of the 10th year has no bearing on the actual yearly maintenance cost, and certainly wouldn't make the total maintenance cost over the term lower... you still have to cover those COSTS before you realize a residual value. I mean your final overall economic standing would change... but not the cost.

Do you guys disagree?

 

[IlPadrino]

I agree with the problem solution as written... and I suggest you're trying to read too much into the question (even if I agree you're reasonable in what you're reading into it if this question were a real-life problem).

The question asks for the "present value or present worth of the highway costs, ignoring user costs". Residual value should clearly be included in the present value assessment when you're comparing alternatives, which could very well be what this question is about (not "amount of money that they needed to put away to cover all costs associated with a 20 year life" as you assume).

You were given no good stated reason to exclude the residual value and in this case, I think adding assumptions can only hurt.

 

[petobe]

I agree with the problem solution as written... and I suggest you're trying to read too much into the question (even if I agree you're reasonable in what you're reading into it if this question were a real-life problem).
The question asks for the "present value or present worth of the highway costs, ignoring user costs". Residual value should clearly be included in the present value assessment when you're comparing alternatives, which could very well be what this question is about (not "amount of money that they needed to put away to cover all costs associated with a 20 year life" as you assume).

You were given no good stated reason to exclude the residual value and in this case, I think adding assumptions can only hurt.

But thats just it - the problem isn't about comparing alternatives... it is about determining the highways costs over the life of the pavement. Or at least that's how I interpreted it... I do agree, though, that the assumption that the information they want could be used in a comparative analysis - and that assumption is just as reasonable as the one I made... I do think it is ambiguous though. I actually did the problem first including the residual - and then read what they wanted again, and said to myself "oh, they only want the costs - i have to re-figure it for that" and took the residual value out of the equation. I figured it was one of those NCEES tricky things where the wording they used was actually directing you to the answer they want (because I have run into other problems that use similar methods)

dang it.

 

[IlPadrino]

It's like Occam's razor... never make more assumptions than you need in order to solve the problem.

For what it's worth, I don't think NCEES uses the concept of distractors much, if at all... they're not trying to "trick" you.

 

[Road Guy]

the reason they include it is because the formula is based on pure economic theory and not "engineering", its based on what the overall cost / savings will be at the end of the life on doesnt really look at it from a common sense standpoint.

But its widely used throughout the planet so it must have some merit.......

 

[wilheldp_PE]

For what it's worth, I don't think NCEES uses the concept of distractors much, if at all... they're not trying to "trick" you.

You are correct that the questions are not worded to mislead you. However, question writers for the various exams do intentionally make common mistakes and math errors in their solutions to come up with the incorrect choices on multiple-choice problems. I know this first hand because my dad used to write questions for the IE exam.

 

[petobe]

You are correct that the questions are not worded to mislead you. However, question writers for the various exams do intentionally make common mistakes and math errors in their solutions to come up with the incorrect choices on multiple-choice problems. I know this first hand because my dad used to write questions for the IE exam.

yeah, this is what i meant... not a trick to make you get the wrong answer - but i thought they were using specific wording to gear me towards an answer (ie.. the cost only)

 

[IlPadrino]

You are correct that the questions are not worded to mislead you. However, question writers for the various exams do intentionally make common mistakes and math errors in their solutions to come up with the incorrect choices on multiple-choice problems.

Yes... it would seem many problems have answers you could reasonably (but incorrectly!) get with even just a minor oversight. This is why the Number ONE recommendation is to always check your units.