Tag Archives: scheduling tools

Schedule Calculations – Old and New

CPMThe way CPM schedules were calculated in the 1970s and 80s (prior to the availability of low-cost PC scheduling tools) used a simplification designed to minimise error and speed up a tedious task.  Whilst some of us are old enough to have used this ‘manual’ technique on real schedules, everyone in the modern world recognises Day # 1 = Wednesday 1st October and a 3 day duration activity will work on Wednesday, Thursday and Friday to finish on the 3rd October and the fact 1 + 3 = 4 is simply an anomaly in the way integers and ‘elapsed time’ interact that has to be dealt with inside the computers computations to produce accurate date based bar charts and tabulations.

Unfortunately there has been a rash of postings on linked-in over the last week totally confusing everyone with their nonsense about CPM calculations.  This blog is designed to correct the message!

To overcome the problem of a 3 day activity starting on the 1st October finishing on the 3rd October, but  staring on day 1 and adding a duration of 3 gives you 1 + 3 = 4, the simplified manual calculations assumed the starting point was ‘day Zero’ 0 + 3 = 3!

However, the old manual calculations starting from day Zero have never been correct – the start day number for every activity in a schedule is always the day before it actually starts.  The end dates (day numbers / dates) are correct and the advantage of this option is it only requires one simple calculation per task for both the forward and back passes and the Free Float calculations are a simple subtraction.

EF = ES + Duration
LS = LF – Duration ….  Easy!!

This simplistic methodology was absolutely essential for manually calculating large PDM schedules. The ‘normal’ scheduling practice through to the mid 1980s when affordable PCs arrived – very few companies could afford the expense of mainframe scheduling tools and those that did wanted to make sure the data was correct before the computer run.

The accurate calculation used in all scheduling software, recognises that a 3 day activity starts at the beginning of day 1 and works on days 1, 2 and 3 to finish at the end of day 3 and its successor (assuming a FS0 link) starts at the beginning of day 4.  Unfortunately these ‘real’ calculations require much more complex calculations[1].

ES = 1, EF = (1 + 3) – 1 to get to the end of day 3.
The Zero duration link requires (EF 3 + 0) + 1 = the next activity ES is the start of day 4.

This approach more than doubles the amount of calculation effort and increases the opportunity for error and of course affects Free Float calculations as well.

Fortunately computer software is not prone to making calculation errors and runs these more complex sums 100% accurately to calculate the date activities start and end accurately when transposed onto a calendar. For more on the actual calculations see: http://www.mosaicprojects.com.au/PDF/Schedule_Calculations.pdf

Given no one has used manual calculations to determine a major schedule in the last 20 years (at least) the old simplistic manual approach is redundant and should be consigned to my area of interest, the history of project scheduling (see: http://www.mosaicprojects.com.au/PM-History.html).

[1] For a more complete discussion see the excellent paper by Ron Winters written in 2003 and entitled ‘How to Befuddle a College Professor’, which can be found at:  http://www.ronwinterconsulting.com/Befuddle.pdf

Advertisements

Time Analysis Schedule Calculations

There are a range of options for the calculation of dates and float in a CPM network.

I’ve just finished a White Paper focusing on the basic calculations and would appreciate comments on the correctness of the calculations and the methodology adopted.  The aim is to produce a definitive document that is generally agreed. 

You can download the paper from http://www.mosaicprojects.com.au/PDF/Schedule_Calculations.pdf   All comments gratefully appreciated.

Scheduling Tools

Has Microsoft overcooked the price and performance of Microsoft Project (MSP)? With the impending release of Project 2010 most organisations should be re-evaluating their scheduling tools. Blindly following the Microsoft upgrade path should not be an option.

The trigger for this post is a number of emails I have received plus comments in a number of published articles and on Planning Planet.  Some users criticise MSP for flawed analytical performance, poor data handling and lack of real power in analysis. Other users criticise MSP for being too complex and too hard to use (you could almost feel sympathy for the MSP development teams dilemma). These criticisms have not changed much since the release of Project 2003 and Project Server. What has changed dramatically is the scheduling software market.

Through to the early 2000s Microsoft virtually gave MSP away, almost anyone could access a ‘competitive upgrade’ for under US$100. The very low cost of MSP effectively destroyed 90%+ of the mid to low end competition, TimeLine, CA SuperProject and a host of other businesses closed merged or changed focus.

Today most people outside of major corporations pay around US$1000 for a set of MSP. This tenfold increase in the ‘real price’ of the tool, primarily caused by the elimination of heavy discounts has opened the window for a host of new players in the mid to low end scheduling market place. Many with free options.

Asta PowerProject seems to be a complete replacement for MSP with equivalent levels of capability and sophistication and better presentation and analytical capabilities.

Other graphical tools include CASCAD-e and NetPoint

Some of the tools that are completely free, or have free entry level options include: jxProject, Gantter.com, PlanningForce and OpenProj

This is not a comprehensive list by any means more tools are documented on our ‘scheduling home page’. And I have not ventured into discussion of the high end products such as ACOS, Micro Planner, Primavera, Spider and the Deltek range.

The purpose of this blog is to challenge every organisation to really evaluate their scheduling requirements and test the market before letting their IT department blindly follow the Microsoft upgrade path.

Project 2010 may still be the best answer, but this needs to be an informed decision based on a proper review of the available alternatives. Simply paying the cost of upgrading to project 2010 (including licence fees, retraining and data conversion costs) without re-testing the market should be seen as being totally unacceptable because in 2010 there is a real choice of tools available!