operations research Archives - Page 4 of 13

Resource Leveling Techniques, and Why Yours Guesses

Search for resource leveling techniques and you get lists: level within float, delay non-critical tasks, split tasks, reassign resources, adjust the calendar, fast track, add people. All real things you can do. None of them is what the button in your scheduling software is actually doing when you press it, and the gap between the list and the button is why two tools level the same plan and hand you two different schedules.

Underneath, there is one technique with two moving parts, and a lot of arbitrary choice in the second part. Here is what it did to a 21-task renovation.

Horizontal bar chart of the eleven tasks leveling pushed later, from Driveway and Landscaping at 14 working days down to six tasks at 3 days each. — Five of the eleven tasks that moved had no CPM float at all. A technique that only shifts slack tasks cannot produce this schedule.

Continue reading “Resource Leveling Techniques, and Why Yours Guesses”

Resource Loading: The Chart Under the Gantt Chart

A resource loading chart is the histogram that sits under a Gantt chart: for each day of the plan, how many people that day needs. It is the least glamorous drawing in project management and the only one that ever told me anything I did not already know from looking at the bars.

Here is one for a 21-task house renovation, before and after the schedule was made to fit inside the crew that actually exists. The two panels hold exactly the same amount of work.

Two resource loading charts of the same renovation: before leveling the daily crew count peaks at 5 and exceeds the cap of 3 on 14 days; after leveling it never exceeds 3. — Same 155 crew-days under both panels. Leveling changes the shape of the histogram, not its area.

Resource Leveling vs Resource Smoothing, Measured

Resource leveling vs resource smoothing is usually taught as a pair of definitions to memorise for an exam: leveling may push the finish date out, smoothing may not. Both true, both useless the moment you are standing in front of a schedule that needs fixing, because neither tells you which one your plan can actually give you.

They are not two techniques. They are one trade-off, read from opposite ends. I solved the same 21-task renovation both ways and the two answers landed on exactly the same curve.

Line chart: with 3 general crew the renovation finishes 2 December, with 4 it finishes 30 November, and with 5 or more it finishes 25 November. — Resource leveling picks the left end and lets the date move. Resource smoothing wants the right end — and on this plan that means five people, not three.

Continue reading “Resource Leveling vs Resource Smoothing, Measured”

Fast Tracking vs Crashing: I Measured Both

Every project management course teaches fast tracking vs crashing in the same order and with the same moral: if the date is at risk, fast track first, because overlapping tasks costs nothing but risk, and crash second, because crashing costs money. I have repeated that advice myself. Then I put both through a solver on the same plan and got the opposite of what the order implies.

Fast tracking pulled eight working days off the schedule and one day off the job. Crashing pulled six off the schedule and eleven off the job. Same 21-task renovation, same solver, same afternoon.

Grouped bar chart: fast tracking pulls 8 working days off the finish with unlimited crew but only 1 with the real crew, while crashing pulls 6 on paper and 11 in reality. — Fast tracking wins on paper and loses on site. Crashing is the only one of the two that gains more in reality than it does on the network.

Project Crashing: How Many Days Can You Actually Buy?

Project crashing is what you do when the date is wrong and the work is not going to shrink on its own: you pick tasks and do them a different, faster, more expensive way. Rent the spray rig. Order the wall panels prefabricated instead of stick-framing them. Hand the drywall to a subcontractor. The version in the textbooks is a tidy table of crash cost per day, and in twenty years of doing this for a living I have never once had a job hand me that table.

So I ran it properly instead of describing it. Same 21-task house renovation I have been using all series, three general crew, six alternatives a real builder could actually buy, and a solver that has to choose which ones are worth it. Here is what crashing bought, change by change, with nothing rounded in my favour.

Step chart: with no changes the renovation finishes 2 December; subcontracting drywall alone pulls it to 25 November, adding prefab framing to 19 November, and adding a roofing subcontractor to 17 November. — Every step down costs one more argument with a subcontractor. The first one buys the most days.

Continue reading “Project Crashing: How Many Days Can You Actually Buy?”