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How to display Resource Histogram by Month with Men Required

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David Ball
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Hi,

 

In the Resource Usage Profile I am trying to display the histogram by Month with the number of men shown down the left hand side rather than the hours for the month.

I display the columns by Month by setting the Timescale to Quarter/Month.

I then need to enter a figure for hours per time period in the "Divide intervals totals by" box in the Graph tab of the Resource Usage Profile Options dialogue box.

 

This is where I have a problem. If I set up for weeks it works fine. I display the columns by week and enter 65 into the "Divide interval totals by" box (we work 65 hours a week). This works. But when I try to do it by month I have a problem. The months have differing numbers of days, so what figure do I enter?

Also, what does the "Unit of Measure" box do? I have tried entering W, M , Hr. Nothing seems to make any difference.

 

Thanks very much 

 

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Rafael Davila
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In my domain we have many resources that are always busy.  To keep these resources busy is the easy part; the issue is about creating reliable resource models that will get it right.   One such resource is a tower crane on a building.  There is a limit on how many cranes you can put on the building.  There is a limit on how much work a crane can do on a single day.  Frequently there will be more activities competing for the crane than the capacity of the crane.  Assigning the crane to a hammock will not level it, partial assignments will do it. 

Because of this need software developers created functionality that allows for modeling partial workloads.  That many missed the math is another issue, a very dangerous one because it not always is obvious, at some point it might not show but as the schedule progress different to as planned the flaw might surface.

Of course a good plan will target for efficient use of resources as much as possible but an unfeasible plan is by no means a good plan.

For over 20 years Spider Project programmers realized that in order to always get right resource leveling on partial workloads it was necessary to distinguish between resource quantity and effort.  Same as most others they realized there is a real need for partial workload assignments, but they got it right from the very beginning.

Rafael Davila
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There is no need to be in denial, it is not that difficult, just model one of the simple scenarios previously exposed and see it by yourself!  It is easier than resource leveling the beds on an oil rig [so simple] and never missing it.

David Kelly
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You shouldn't be so hard on yourself, Rafeal!

Rafael Davila
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There Are None So Blind As Those Who Will Not See

 David,

I was involved in planning of many resource constrained projects and many of them needed moving resources to remote places to do the job. So we optimized resource constrained schedules trying to find the minimal amounts of resources that are sufficient for meeting project targets. And still I don't remember a single schedule where all resources were busy all the time on the project works. In projects activities depend on other activities and they cannot be performed at any time. Specific resources may wait until preceding activities will be finished and this is not rare. The work itself changes too and requires different resources at different time periods. I can suggest a lot of examples and I am sure that you can suggest no less.

David Kelly
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Vladimir,

 

90 per cent or more of the resources on any asset are supposed to be busy all the time. 

We need to aggregate operations, projects, maintenance to get a bed count. About 25-50000 activities depending on the size of the asset.  

 

I have a little slack time so I shall do some analysis on the metrics we use. 

David, it may be true for operations but not for projects. You wrote that planned projects with the number of beds constraint. So you needed to reserve a bed for a person that was not busy all the time, isn't it? It is common sense, no need to ask Primavera planners if this is correct.

David Kelly
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I have started a wee project to investigate the legitimacy of saying that 100,000 manhours divided by a CPI equivalent of 7 productive hours a day = the number of men.

I have asked enough lead planners to project control managers to get data from over 500,000 maintenance work orders currently live in P6 across a dozen companies. 

I accept that just because we have done it this way for forty years does not mean that it is correct.

Personally, I think it is - but I have not done the maths. 

Rafael Davila
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Vladimir,

What surprises me is that self proclaimed experts cannot figure it out.  Maybe in a few decades they will learn how to count resource quantities using appropriate units such as 1 resource different to effort such as man-hours.

Primitive Counting photo Caveman Counting_zpsj6vdx88r.png

 

I would not be surprised if database developers cannot understand it but that many others such as people in academia and operations research scientists have not warned about this flaw is UNBELIEVABLE! The flaw seems like not exclusive of MSP and P6, it looks like much older, from the beginning of the times when CPM was expanded to include resource leveling.  Perhaps some software developers know about it but it would mean they acknowledge their models are wrong. 

 animation frog icon netpac  dancing party FrogDance-blank_molly.gif

WOW!  LOL

Zoltan,

I many times suggested an example when two resource units work with 50% workload (4 hours per day). Isn't it the same as one resource unit that works 8 hours per day if to judge only by manhours?

When we plan resources we want to know the number of resource units that shall be used on the project works at any time period. We need to know the number of beds, capacity of the temporal camps, how many machines shall be moved to the working site. “An average” is not sufficient.

An average makes sense only when all resources are busy all the time on the project works. In practice resource requirements are not exactly the same each day and month. It is necessary to know maximal requirements and try to smooth peaks if possible minimizing required resource quantities.

When resource requirements are different at some days of the month the number of required resources is larger than “average” in any case (even if to ignore part time assignments). That is why “average” is minimal resource requirements.

 

Project model is decision making tool. If it does not answer to questions like what if the number of available resources will be larger or smaller then it is useless for resource management and decision making. If the leveling never works for you look for more reliable software.

Zoltan Palffy
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Vlad average does mean that all resources are working full time during the time period. 

Resource assignment in hours can show resource quantities by dividing the manhours by the time interval be it day, week, month, quarter or year. Thats the WHOLE point. 

No one is talking about leveling here and leveling never works anyway.

Zoltan, average does not mean that all resources are working full time during the time period. It looks more like minimal than average.

Resource assignment in hours do not show resource quantities. More than that, without taking into account required quantities resource leveling is not reliable and produces not realistic schedules.

The problem exists and it is more serious than just wrong reporting.

Zoltan Palffy
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everyone is splitting hairs here and is losing the site of the big picture we are taking about the word average here which means. Average means a number expressing the central or typical value in a set of data, in particular the mode, median, You are trying to get an EXACT number the tool is meant to do this.We are talking about months that have 29, 30 or 31 days so at the most it a 3 day difference. If you really want to do this go to Project Resource Assignments and highlight this data then export to excel and make the minor adjustments there per month 

Rafael Davila
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A schedule that does not meets any resource constraint is not feasible, in your case number of beds is one such limitation, perhaps not as interesting as the issue on production rates that must also be considered.  Well it seems like correct resource leveling of number of beds is an easy task.

Rafael Davila
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David,

Surely we can continue latter.

In my schedules we call Beds in Spanish "Camas" to elevated slab flying forms we use for concrete buildings and these are spatial resources, similar to the sleeping beds on your job, just that many schedulers are not aware of this.

In the video you will se how I changed availability of Flying Forms with a simple change in quantity of material production, I also delayed one building as to force other building to take its place.  Manual resource leveling if using "soft links" would take a lot of effort and would be error prone, if using spatial resources a few clicks will do it.

Elevated Slabs photo Elevated Slabs_zpspzo4ll1h.png

 

CR2689 photo CR2689_zps2522bej3.png

     If using spatial resource modeling the software will get it right.

David Kelly
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very ineresting, Rafael, and I will have lots to say about your post later (I hope) - But some work has come in!  So I shall go and earn some money for a week......

Rafael Davila
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Vladimir,

It looks like P3 as well as Asta PP use resource per time period without considering resource quantity and therefore are equally flawed.

In addition same as P6 both are poor at handling team work and might allow for different resources working at different time when required to work together as a team.  I am not sure if this additional flaw also exists in P6, I am aware of the flaw in P6 due to lack of a quantity field different from effort field, fields that might look similar but that are different.

Unbelievable you can assign 10 hours to the car and 5 hours to the driver when they must work as a team!

Like many I did not noticed the flaw.  It became obvious after you warned about it.

http://www.planningplanet.com/forums/asta-powerproject/576477/asta-pp-re...

Best Regards,

Rafael

Resource Quantity photo ResourceQuantity_zpslqyhgktv.png

Thank you David!

We have the same problem when import project information from the estimating software. For activity cost estimating manhours matter but the number of men does not. So we try to understand what resource crew shall be used on activity execution. It helps if there are leading machines. In this case we set activity duration equal to hours of the leading machine and calculate other resource quantities and workloads dividing their hours by hours of the leading machine. It is primitive but yes, if other information is not available there are no other options.

But when our team members know the technology or when we can get the answers from the specialists we create and assign real resource crews and use volumes of work and expected resource productivity for activity duration calculation. Resource hours is not initial but calculated data.

In corporate project management systems there are usually corporate reference-books (databases) that include resource productivity on different activity types, standard resource crews on different activity types, resource workloads on different activity types, etc. When such reference-book library is available then it is sufficient to enter activity type and volume and get everything else from the reference-books. Estimating data is used as the parallel estimate of the same work for comparison.

Rafael Davila
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Bed availability can be modeled as a spatial resource for Beds never be overloaded, easy just resource level your spatial resources.

  • http://www.pmknowledgecenter.com/node/104
  • Spatial resources are required by a group of activities, rather than a single activity as renewable resources. The spatial resource is occupied from the first moment an activity from the group starts until the finish of all activities from that group.
  • https://www.stottlerhenke.com/news/press-releases/us-navy-selects-aurora...
  • Submarine maintenance schedules must satisfy complex resource constraints. While submarines are at the NSSF, the Navy must perform many maintenance operations that each require different combinations of resources, such as maintenance shops, equipment and maintenance teams with specific skills and certifications. Large submarine parts are removed from the submarine and transported to the 35 specialized maintenance shops where they are reconditioned or repaired. Because space in the submarines’ narrow passageways and access to the submarine hatches is very limited, the scheduling system must also reason spatially, so the Navy can schedule transports in a way that minimizes interference with the submarine’s other activities and the movements of its crew.
  • Maintenance shops as well as the submarines’ narrow passageways and access to the submarine hatches can be modeled as spatial resources, cannot be modeled if using traditional resource types.
  • In Spider Project we model spatial resources via consumable resources consumption/production.

http://www.hse.gov.uk/offshore/notices/on-82.htm

  • 7. There should be sufficient beds or bunks for the number of people expected to sleep on the installation. No person should be required to share a bed (hot bunking). Changing the mattress and/or bedding to allow two people to share a bed or bunk is considered as 'hot bunking'.

The specialist is a traditional resource whose availability can be modeled in a dynamic way if using resource production/consumption by specific activities. Can be the start of Fly-in and the finish of Fly-out.

Fly-in and Fly-out as specified is not a periodic activity so it shall be manually scheduled.

Beds0234 photo Beds0234_zpsdo4e3d8j.png

I do not agree with the statement - So P6 can work with man-hours and the difficult relationship with number of men. If there is no field for resource quantity there is no way P6 can truly relate man-hours to resource quantity in the presence of partial assignments and work calendars with different work hours per day.  In addition the example provided by Vladimir on why P6 resource leveling is unreliable is just one of the many possible situations where P6 yields unfeasible resource leveled schedules. Another example can be when you have a work week with different work hours per day, under such scenario average man hours per day will not make it. 

It is amazing how many schedulers cannot realize how flawed P6 resource leveling is.  Some do not even know about spatial resources, consumable resources, financial resource leveling, variable quantities and variable workload, resource production rates and many other resource planning functions that cannot be tackled using the traditional renewable resources.  For decades these functions have been available in more advanced software.

Man-hour production rates are very useful but not always enough, the crew composition do matter, this is why RS Means provides you with the crew composition, the same number of workers but different excavation equipment will have different crew production rates. I know some in the oil industry use Gulf Publishing books on man-hour production rates but I never believed it to be good enough, it is up to the industry to demand better.

For the tracking of resource productivity I have a preference on Financial/Accounting software capable of tracking unit production cost and production rates.  For when financial resources are limited and must be leveled then the CPM schedule is the tool of preference, if funds become depleted then jobs will be stopped or in some cases planned with no interruption, for this a good scheduling software capable of considering financial constraints is a must.

http://www.planningplanet.com/forums/spider-project/594355/time-money

David Kelly
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Fascinating discussion, I have been a way for a few days actually working which is a nice change given that I work in Oil and Gas which is a wee bit quiet at the moment.

Of course, everything Vladimir says about FTE not being translatable to number of actual men by dividing manhours by a constant is true, BUT:

I live in a world where the scope of work estimate is always in manhours. These are almost always what our American cousins call Wrench Time. No meal breaks, no permit delays, no walk-to-work.

Contractors have sophisticated Work Management Systems (WMS) that have norm based estimating systems within them that create a manhour content that is NOT FTE compliant, e.g. what does 47.45hrs of Pipefitter, and 17.32 hours of Rigging actually mean in terms of number of men on the job? I have never encountered an estimating system in my Oil, Nuclear, Defense, Ship Building, or Renewable worlds that tells me “how many men”.

For forty years what we have done next is to impose a methodology that helps us understand the impact of non productive time. We do not have “resource efficiency” or anything like it in P6 – so we either:

  1. Reduce resource availability (e.g. I’ve got 6 electricians, so I will tell P6 I have 4)

  2. Factor the manhours (e.g. this job is at the flare tip, needs three permits, a LONG climb to work – lets multiply the manhours by 3)

  3. Reduce the number of working periods in the scheduling calendar. (e.g. We are paying for 12 hours, but on a satellite platform we only get 4 working hours per day.)

Option 3 is universal as it preserves the manhour content in the estimate which option 2 makes irreconcilable, and does not increase my FTE algebra as option 1 would.

So I am left with Wrench Time and an activity calendar that seeks to match CPI. Dividing one by the other to approximate FTE is the best we have got. Yes, we could ask the maintenance manager to code some FTE information on the 25,000 PMRs he has per asset, but he would probably just ask us to change our medication. The obsession with CPI in my wholly reimbursable world makes it the commercial divisor of choice.

There has been a breakthrough for the asset owners in the FTE debate. The asset owners I do most of my work for are the operators of offshore oil and gas installations. Having more men on the platform than there are beds (strictly speaking, seats in the emergency escape vessels) is illegal. The common logistics package which monitors all movements of personnel, helicopter flights, crew rotas only knows about number of men. We must have exactly a full complement of men on board at all times. Dividing P6’s manhours by anything is no way to meet the statutory obligations. The interface between P6 and the Vantage logistics system has been a tortuous path through exquisitely complex spreadsheets.

Let us take a worse case example from Vladimir’s complaint about FTE. We have to send a specialist offshore to do a job estimated at 4 manhours. We need a flight out and a flight back and a bed for all the time he is there. Only the logistics system knows the flight dates and hence the duration of the activity. The system at the early adopters allows someone using the P6 Window’s client to send a request for an offshore visit directly to the queue of the logistics system, and when successful it writes back the date of the fights in and out – and a new resource assignment “Bed” for every day the specialist is offshore to do the 4 hour job. If our specialist’s return flight is delayed, then the Actual FTE is more than the Budget FTE.

The bit I like most is when the crew swipe through the heli-deck with their ID cards, the Resource Availability in P6 is updated.

This means all of the really difficult FTE sums are done in the logistics system, and the results written to P6. P6 behaves perfectly well with this data – I would just HATE to have to type it into the Resource Assignment table myself. 

So P6 can work with manhours and the difficult relationship with number of men. It just needs some help with the maths. P6 is light on estimate-and-approval, and I put “The Valdimir problem” in that category.

Now whether this whole issue is for “scheduling” software to solve or the WMS is a rant for another time…….

 

 

 

 

 

 

Zoltan, as I wrote earlier Spider users do not need to divide anything. They get direct reports on resource quantities and hours. I am not sorry that they have this opportunity and enjoy correct resource management.
Rafael Davila
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in P6 we use FTE

  • Then P6 users are doing wrong math.  What it is amazing is how some P6 users cannot see the trap even after it is called to their attention.

If you have assigned 16 man-hours of resource A in one month; sixteen divided by 172 yields 0.0930023 which is meaningless.

  • It might be you need two resources A working together the same day for 8 hours or it might be you need just one resource A working two days for 8 hours each day. Or it might be you need four working as a team for four hours, in such case one single resource working for 16 hours will not make it as in order to do some work he needs the assistance of the other three.

If you have assigned one man-hour of resource A in one month; one divided by 172 yields 0.0058139 which is meaningless, if you divide it by 8 if looking at a dally distribution you will get 0.125 which is also meaningless.

  • It might be you need four resources A working together the same day for 15 minutes.

P6 resource histrograms are not a reliable way to determine resource quantity needed!

While full-time equivalent employees (FTE) is used for some accounting purposes it shall never be used to determine quantity of resource requirements.

The flaw in P6 resource leveling not always surfaces out when resource leveling the schedule but it does without any warning as reported by others.

Zoltan Palffy
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Vald this is a Primiver P6 forum so in P6 we use FTE sorry spider does not 172 is the answer yesterday, today and tomorrow. 

Zoltan, I do not recommend anything special because do not know P6 as good as you and David.

I expected to learn about some workaround but it looks like it does not exist and P6 resource planning and reporting are not reliable.

We do not have this problem because Spider Project plans and reports both resource quantities and resource work hours.

 

 

Zoltan Palffy
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Vald

so what do you recommend using # of flies ?

Zoltan, FTE is not an industry standard in my country. Fortunately we do not accept follies as industry standards.

And besides if something is totally wrong why to use it even if it is an "industry standard"?

Do you have other arguments except "this used ALL of the time"?

Zoltan Palffy
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VALD

do you understand that FTE is an indusrty standard so why not use it so stop with the monkey wrenches and red herrings.

this used ALL of the time. 

Rafael Davila
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Vladimir,

I know about this issue but I do not have P6 license as to showcase it. I wonder if Microsoft Project is equally flawed. I also know about Asta PP resource leveling flaw that will allow resources working on a single activity to be assigned at different times when the usual requirement is to keep them working as a team.

The devil is in the details, you will not get rid of it by simply pretending there is no devil.

Best Regards,

Rafael

 

Devil008 photo devil8_zpst36gr8oa.gif

Rafael,

your example shows why FTE leads to wrong conclusions on monthly reports.

But the problem is much worse. P6 does not consider resource quantity and as the result may produce wrong resource constrained schedules.

Let's suppose that in my example in this discussion P6 user set availability of workers as 16 hours per day that looks equivalent to two units.

In this case P6 will schedule the project as shown in my post (maximal manhour requirement is only 14.4 per day) though two workers are not sufficient for doing all three activities in parallel. With part time resource assignments required number of project resources may be underestimated and P6 will produce unrealistic project schedules that do not meet resource constraints.

Rafael Davila
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  • FTE is just the man-hours per month divided by a constant, it does not provide any additional information other than man hours divided by a conversion factor. Take a look at the following schedule and tell me what is the use of FTE. All resources are working on same 8HD5DW calendar, it does not provides useful information as to determine how many resources are required.
  • I have never worked on a single construction job where all resources are assigned the same 8HD5DW calendar but lets model such a schedule, for the sake of never say never. 
  • RQ111 photo RQ111_zpsmkzoxj0h.png

Zoltan, do you understand that FTE is misleading?

Zoltan Palffy
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working a standard 8 hour shift per day 5 days per week its still 172 this will give you the FTE

Rafael Davila
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If it is correct to say - "not true the planned number of men required per month is usually based on the number of units which is man-hours which is distributed over the duration of the activity" then you shall have no problem with answering the following question.

I have a job consisting of a single activity with a budget of 24 man-hours and a duration of 3 days to happen within the same month, then what is number of men I used to schedule the activity? The total man-hours is 24, activity duration 3 days within the same month, cannot be any easier.

As a hint I will give you a few of the many possible options that will met the above.
a) One man for 3 days activity durationworking 8 hours per day? [1 man x 3 days x 8 hours/day = 24 man-hours]
b) Two men for 3 days activity duration each working 4 hours per day, same 4 hours per day shift? [2 x 3 x 4 = 24 man-hours]
c) Three men for 3 days activity duration each working 8 hours per day on different days, different shifts? [8 + 8 + 8 = 24 man-hours]
d) Four men for 3 days activity duration; two men working 3 hours a day on days 1 and 2 for a subtotal of 12 man-hours, and 2 different workers but sharing the same skill working 6 hours on day 3 for a subtotal of 12 man-hours, for a total of 24 man-hours in 3 days.[2 x 3 x 2] + [ 2 x 6 x 1]

It is basic mathematical knowledge that given a set of equations if there are more unknown variables than equations the problem is undetermined and there will be infinite number or solutions that will satisfy all equations.

Try it yourself, each activity will use different resource quantities but same activity duration and same man-hours per activity.

RQ878 photo RQ878_zpssfr8oryc.png

Simple example: activities 1, 2 and 3 require a worker with 60% workload (60% of the worker work time). So each of them require a separate worker. If these activities are scheduled to be executed in parallel then they require 1.8 manhours per hour but 3 men to do the job.

Another example: at the beginning of the month 10 people are assigned on project activities, then only 2 persons. Total manhours will not show peak resource requirements.

Below the project that illustrates what did I mean. Activities 1, 2 and 3 are linked with SS dependency with 10 days lag, each requires a worker with 60% workload.

At the bottom resource histograms that show quantities and manhours by months and by days.

You may notice that second month requires more manhours but less quantity.

 

 photo RR_zpshitcwqyr.jpg

Zoltan Palffy
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not true the planned number of men required per month is usually based of of units which is manhours which is distributed over the duration of the activity. 

I have read in the original post: “I am trying to display the histogram by Month with the number of men shown down the left hand side rather than the hours for the month.”

FTE does not show required resource quantities.

Planned number of men per month (week, day) is not based on manhours. Resources may be assigned part time, there my be idle times between activities where certain resources are used. Resource quantity per period may mean peak quantity and applying resource leveling these peaks may be smoothed.

It looks like P6 just does not plan and report resource quantities. It creates problems with P6 resource leveling reliability. I do not mean schedule optimization - with wrong quantities the resulting schedule may be not feasible and required resource quantities underestimated.

Zoltan Palffy
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Vlad please read the original post again what he is looking for here is FTE or full time equivalent men per month. This is a man month this is for planning purposes. This has NOTHING to do with actual number of men worked per month of course actual manpower varies. Then you please tell me without this FTE how would you determine the PLANNED  number of men per month that you need on the job based on the manhours loaded per activity. How else are you going to get a actviities planned over time not overtime I mean over a time period. 

David Kelly
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P6 does have a few issues with resource quantities that are really only obvious if you look at more sophisticated software.

For Example  "Resource Efficiency"  i.e. i pay for 12 hours, how much actual work do I get?  Has occupied some of my time every week for the last 40 years, and P6 really only has 20th century work arounds. You could argue that such work arounds while rather old are at least well tested. They are also quick to apply.  In its defense we have a limited time to guess what to do next. That we use deterministic tools in planning for what is essentially a probabalistic environment is perhaps the greatest of these compromises. We cannot say "stop the project, I have not finished planning it"

Zoltan, for many possible reasons an assumption that total manhours are proportional to resource quantity and that everybody worked 65 hours per week on project activities is not correct in most cases.

Zoltan Palffy
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when it says to use an average for the manpower it uses what values that you have set in the users preference under the Time Units tab under hours per time period. It does not mater that some months there are 29 days or 30 or 31 days per month.

So if you look at how the 172 was derived which are the standard working hours per month based off of working 8 hours per day 5 days a week it is 172/40 or 4.3 weeks per month on an average. So take 65 * 4.3 = 279.5 working manhours per month. 

So when you do your graph per month instead of using the default average so like you did when you were looking at it weekly and over ride the setting and you typed in 65 just type in 279.5 to get average manhours per month and this will give you the number of men required by MONTH based on working 65 hours per week. 

As I know P6 does not have resource quantity field. Manhours and quantity are not the same. Some resources may be used part time, some peaks in resource requirements may last short time, resource calendars may be different, etc.

So an approach when total manhours are divided by 8 (10 or any other estimate of the work day duration) for calculation of resource quantity is not correct. More than that - resource leveling may produce wrong results when the software levels hours and not quantities. The requirement to use 2 resource units with 50% workload looks the same as using one resource unit full time.

Reports that show resource requirements by months are usual in our practice.

David Kelly
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Joined: 19 Oct 2004
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Hmm, you can’t do what you want, and I am unclear why you would want it……

 

If I have four activities on a finish to start relationship, the first lasts exactly January, the second February, the third March and the fourth (imaginatively enough) all of April, I have four activities that are “One month” long and have three finite durations between them.

 

If I then add a resource to each of the activities, lets keep it simple one person working full time, I have three different budgeted units across the four activities.

 

The Calender I use for these “Task Dependent” activities has “Time Period” settings of 8 hours for the day, 40 hours for the week – and then I am stuck. What is the hourly translation of a “Month”. There is only one value held and four possible values ((28,29,30,31)*Hours per day). So you cannot see a “smooth” histogram if I aggregate by month and want to see average utilisation by month because my four monthly Budgets share three different values and only one divisor…….

 

BUT if I want to know “how many men I need this month” I can just use the “weekly” figures instead? If I aggregate the four months over the roughly 16 weeks, then all of my 16 sub-totals are divided by 40. This is the same number of men as a monthly sub-total divided by the correct amount (which I do not have access to)….

 

Unit of measure is a label for Material resources ( e.g. foot, m3, pair etc) it has no arithmetic impact. Just a lable.