In Deltek Cobra there are three options for weighting the spread of hours when resources are assigned to work packages or control accounts. These are: ‘Linear weight’, ‘Weight Using Hours’ and ‘Weight using working days’. The question is what effect do these settings have when making a resource assignment and indeed what calculations is Cobra performing for each of them?
This article explores the three spread weight options in Deltek Cobra and how they influence time-phased values when resources are assigned to work packages.
Spread Weight Options
The spread weight options can be set during the creation of a project, or in the General tab of the Project Information dialog for an existing project.
These settings will influence the way hours get spread on a work package or control account when a resource is assigned. This is true for manual assignments and also when loading the project data in from a scheduling tool or CSV file.
The ‘Linear weight’ option
This is the most straightforward of the options. Deltek Cobra will spread the total hours to be assigned evenly across all the periods in the work packages range. Let’s look at a simple example.
In the following figure you can see 300 hours of Engineer time has been assigned to a work package. The project is using the Linear Weight option. The work package starts on October 1, 2015 and finishes December 18th, 2015, spanning 2 full months, and one partial month.
Highlighted in green above, you can see that with Linear Spread as the default setting for this project, Cobra has simply divided the 300 hours by 3 periods, giving 100 hours to each period. Cobra has ignored the fact that December is not a full month, producing a flat profile. This has also skewed the FTE value so this first option is probably not typically what you might want to use.
The ‘Weight using hours’ option
The ‘Weight using hours’ option tells Deltek Cobra to weigh the budget spread based upon the hours in the project calendar. This gives a more accurate picture of the hours required for each period than the previously discussed option. Let’s see what Cobra does with our previous example when the resource is assigned in a project using the ‘Weight using hours’ option.
This time we can see a more realistic spread of the hours, based upon the hours found in the calendar between the start and finish dates for the work package. Cobra generates the spread by looking at all hours for the period, including weekend and holiday hours, that fall between the work package dates. That is to say, it pays no attention to working or non-working days when it’s calculating the average hours per day part of the equation.
Then Cobra assigns the averaged hours per day value to the total number of actual working hours in that period (now taking holidays into account) to give the final number. The math for getting the average daily hours looks something like this:
Average Hours per Period = Budget Hours / Total Hours in date range (weekends and holidays not included).
For our example work package, the dates span a range of 79 calendar days, which adds up to 1896 hours in total. Note this has ignored all non-working periods and used a 24 hour day [79 x 24 = 1896]
Next Cobra divides the budget hours by 1896 to get an average hours per day value.
300 hrs / 1896 = 0.15822785 Average hours per day.
This average value is then multiplied by the total hours in each period according you the project calendar file and date range of the work package.
Oct: 744 hrs X 0.15822785 = 117.72
Nov: 720 hrs X 0.15822785 = 113.92
Dec: 432 hrs X 0.15822785 = 68.35
You may have noticed that this example is showing slightly different results than those calculated by Cobra in the previous figure. Well spotted. That’s because the spreadsheet I used to generate these raw figures doesn’t model every nuance of the Cobra algorithm. There are some fractions of a day that Cobra rolls forward, typically leaving the remaining odd hours in the last period, depending on the work packages span. I didn’t attempt to decompose this part of the model.
Okay, rather than get wrapped any further round the mathematical axle, let’s talk about the practical usage of this setting.
When would you use this setting?
The ‘Weight using hours’ option typically gives you a somewhat smoother spread curve because it’s averaging the hours per day for the entire date range, and then assigning those hours based on the productive calendar hours in that period. It will also give you a slightly smoother FTE spread as a result when the date range doesn’t fall exactly on full periods. However, it’s not quite as accurate as the final option we’ll discuss next.
The ‘Weight using working days’ option
This is my ‘go-to’ option, as it seems to be more precise than either of the other two. Its definition in the Cobra help file is as follows:
“This option instructs Cobra to calculate how many working days fall within the start and finish dates, taking holidays into account.”
It’s more precise because it takes into account holidays within a date range when calculating the average working hours for the work package. Frankly it’s easier to understand and can be exactly modeled in a spreadsheet to boot.
Here are the results’ using ‘Weight using working days’ on our previous example.
A different spread is evident when ‘Weight using working days’ is used on our example project. Cobra has looked at the total working hours in the date range and generated the average working hour value; in this case 0.6578; i.e. it looked at only 456 hours as opposed to the 632 hour we saw previously. Dividing the 300 budget hours by 456 gives the total of 0.6578. Multiply this by the number of working hours (accounting for weekends and holidays) and you get the above spread.
Here’s my test spreadsheet that gave identical results as I worked to simulate Cobra’s ‘Weight using working days’ calculations:
In here you can see the average working hours per day, and then a simple multiplication of that value by the number of working (productive) hours for each period. It exactly matches the results seen in Deltek Cobra.
When should you use the ‘Weight using working days’ option?
Probably always. This setting is definitely more precise than either of its counterparts.
The following table of results show the differences between each of the spread weight options presented in this article.
As always there are many variables to consider when thinking about spread weight options; the EVT, the work package duration, the type of resource being assigned, and so on. Thus it’s hard to pigeonhole these three options for one or another usage. I generally use the ‘Weight using working days’ option and haven’t had cause to regret that yet. I hope that the examples have provided you with enough information on which to make a decision when next you are setting up a project.
Last, but by no means least I want to express a special thank you to Mr. Steven Wong with UTAS. His help in figuring out the underlying mathematical differences between the ‘Weight using hours’ and ‘Weight using working days’ options made this article possible.