When I change the Person Hours per Person Month field on the Accounting tab of the Solution Assumptions dialog, there is no change to the total schedule and effort for the project. Why is this?

Many SLIM-Estimate and SLIM-Control users have asked why their overall schedule does not change when they change the value of the "Person Hours per Person Month" field,  Solution Assumptions screen, Accounting Tab.

The number of Person Hours comprising a particular organization's Person Month is not used to calculate total effort.  If your chosen effort unit is Person Months, the hours per month setting is not used at all. If, on the other hand, your effort unit is weeks, days, or hours, your hours per month will be used to convert the estimated total effort (in Person Months) into your chosen effort unit.

At first, this may seem counter-intuitive. Shouldn't an increase or decrease in the number of Person Hours in a Person Month be reflected in the final estimate?

The number of staff hours in an organization's staffing month is more relevant in the accounting world than it is in software estimation. It attempts to account for items such as vacations, meetings, and administrative activities but in most cases, it is merely an estimate or average.  It may tell how many hours will be charged to a particular project, but has nothing to say about how much is actually accomplished during those hours (or even how many actual hours were worked). The productivity index, or PI, is where actual work produced is measured.

The Productivity Index, or PI, reflects a number of environmental factors such as:

If you are using a PI calculated from your past projects, your user-specified PI already reflects the conditions in your working environment. Holding other factors constant, organizations which have high "administrative overhead" will tend to have a lower calculated productivity index than organizations which maximize time spent working on the most productive project tasks.

SLIM-Estimate® is a macro model based on the software equation described in Measures for Excellence. The SLIM software equation is designed to estimate consistently across a broad spectrum of working environments and accounting standards. For this reason, the effort parameter was originally calculated in Person Years, which are converted to Person Months in SLIM-Estimate. Once the total effort in PM has been determined, it can be converted into your chosen effort unit for display on charts and reports.  But what if you have no historical data and are using the PI suggested by the QSM database? SLIM estimating and project control tools will still give you a good first estimate that you can continue to refine as more project data becomes available. As projects are completed, SLIM tools will calculate the project PI's from your historical data. These PI's can be used to calibrate new estimates to the unique conditions in your development environment.

As you begin to compare and analyze PI's from completed projects, you can spot and eliminate bottlenecks as well as identify successful strategies that you can repeat to improve productivity. Storing project metrics in a useful and accessible form and analyzing them to identify trends is the key to process and productivity improvement. Consistent use of SLIM tools results in increasingly accurate estimates and will help you build a repository of historical data for future analysis and benchmarking.