Can I use the forecast function to explore various staffing options?

When you run a forecast, SLIM-Control bases the future staffing profile on the peak staff and staffing shape defined in the current plan.

While Phase 3 is ongoing, run a tradeoff forecast to see the impact of increasing or decreasing staff and apply any resulting schedule tradeoff to your existing forecast.  To run a tradeoff forecast, select Control | Tradeoff Forecast… from the menu.

Once Phase 3 has completed, use the maintenance forecast to evaluate staff tradeoffs.  The maintenance forecast can only be performed during Phase 4.  To run a maintenance forecast, select Control | Run Forecast… from the menu.

Why don't my staffing rate numbers add up to the same total shown on cumulative effort charts and reports?

First, it is important to understand the difference between the average staffing rate and cumulative effort charts and reports.

Average Staffing Rate charts and reports display the number of FTE staff used by the project during any given month, regardless of how much of the month is covered by that phase. When a phase starts or ends mid-month, the staffing rate for that month is not affected.  

Cumulative Effort charts and reports display the total effort for each month.   So here, when a phase starts or ends mid-month, the total effort reflects those partial months.

To see this more clearly, let's look at an example.  If there 10 people work during December (and the phase begins on December 31st), then the Staffing Rate chart and report will show 10 people for December; but the Cumulative Effort for that month would only be .33 mm (10 people x .033 months). This logic holds true for other rate values like defects and cost.

A second factor that may cause discrepancies between cumulative effort or cost and the sum of the monthly effort/cost rates is really a calculus issue.  Staffing numbers represent the average manpower rate for a given time period. These numbers are typically taken at mid-month. 

For graphing purposes, it is more practical to break up the smooth theoretical curve used to derive cumulative effort/cost into slices (like bars in a histogram) where the top of each bar corresponds to the average staffing rate for each month. Because each bar has a flat top, each month a small amount of error is introduced as there will always be a small area under the curve that is not covered by the bars.

As we add more and more months (bars) to the staffing chart, the bars become narrower and narrower and the amount of error decreases. If the intervals are small enough, the sum of the rates should be very close to the cumulative effort. However, when the intervals are large (for instance, monthly for a 3-month project), the approximation will not be as close. 

The Solution Panel says my peak staff value is 10.3, but the Average Staffing Rate charts and reports show a peak staff of 10.1. What is causing this inconsistency?

The theoretical Rayleigh curve used to derive the values on the Solution Panel is a smooth curve resulting in different staffing numbers at each point along the x-axis. In drawing the Average Staffing chart, this smooth theoretical curve is approximated by a series of discrete staffing rates (with one bar, or average staffing rate value, per month).

The graph and solution panel values won't always match exactly because the graphical peak staff is always taken mid-month for any internal month while the solution panel always shows the highest peak staff point on the staffing curve (regardless of when it occurs). Usually the theoretical curve does not peak exactly at mid-month, so the true peak shown on the solution panel will not match the one on the average staffing charts unless the theoretical peak happens to fall at mid-month.

On large projects where a longer schedule allows a natural smoothing of the staffing curve, the theoretical peak staff will usually be close to the average staff rate value. On shorter projects, you can expect to see larger discrepancies.

When I change the Person Hours per Person Month field in Accounting Options, there is no change to the total schedule and effort for the project.  How can I adjust the estimate to reflect overtime?

The person hours/person month field was never intended to be used for performing time/effort tradeoffs.  For estimating purposes, SLIM-Estimate assumes that a FTE person is a FTE person, regardless of the number of hours in an effort month.  To account for overtime or unusual staffing situations, you should adjust the FTE (full-time equivalent) peak staffing level.  

For example, to reflect 10% overtime, you need a peak staff that is 1.1 times the current peak.  You can do this using the Control Panel peak staffing dial or by extending or shortening the Phase 3 schedule.

NOTE: if you are using a PI from your completed projects, you should only make this type of adjustment if your history does not include overtime.  It is usually better to be conservative in making adjustments as overtime hours are often less productive.  If the completed projects you used to derive a PI had a similar amount of overtime, it is already reflected in the PI and no adjustment is needed.  

What is the impact of selecting various phase staffing shapes? When should I deviate from the default Rayleigh pattern, and how is my estimate affected?

We have found that the default Rayleigh pattern is the staffing pattern that best matches the application of effort to the work to be performed but due to staffing constraints or different software management styles, you may decide that another staffing pattern fits your organization or project better.

In general, the various staffing shapes can be described as follows: 

• Front Load Rayleigh peaks at approximately 40% the phase 
• Medium Front Load Rayleigh peaks midway through the phase 
• Medium Rear Load Rayleigh peaks at approximately 75% of the phase 
• Rear Load Rayleigh (Phase 3 only) peaks at the end of the phase 
• Level Load maintains a constant staffing level over the entire phase
• Exponential (Phase 4 only) gives the most rapid drop off of staffing from the end of Phase 3.
• Stair Step (Phase 4 only) begins at about half of the staffing level from the end of Phase 3 and stair steps down.
• Straight Line: (Phase 4 only) represents a straight-line decrease in staffing from the end of Phase 3.
• Rayleigh (Phase 4 only) is a natural tailing off/continuation of any Phase 3 Rayleigh curve. Not valid if the selected Phase 3 staffing shape is Level Load. 

Level loading is generally seen in the development of very small systems. In larger systems, early application of people often represents wasted effort. The Default Rayleigh shape determines the staffing shape based on the size of the application. QSM has found that small projects (less than 18,000 lines of code) tend to have a Front Load Rayleigh profile while larger systems (greater than 100,000 lines of code) typically reach peak staffing towards the end of phase 3. For very small systems (3 to 6 months in duration with a peak staff of 1-3 persons), a level load profile is more appropriate.

The estimated total effort for each of the staffing patterns for the first three phases will remain constant regardless of the pattern selected. Phase 4 effort will vary, depending on the shape of phase 3. This is because in phase 4, manpower tails off from the final Phase 3 staffing level and is therefore a function of the number of people on board at the end of phase 3 and the length of phase 4. Projects that peak at the end of Phase 3 will have higher phase 4 effort. Projects that peak earlier will have lower phase 4 effort.

If you are using default QSM overlaps for the phases, you can expect to see some differences in the phase overlap when you change the staffing pattern. This occurs because SLIM attempts to produce the smoothest aggregate staffing pattern when moving from one phase to the next.

For example, the overlap percentage between Phases 2 and 3 will be 0% if Phase 3 is Level Loaded. Otherwise the overlap ratio will range from  33% to 40% as the phase 2 staffing shape changes from Front Load Rayleigh to Medium Front Load Rayleigh.