Will the changeover fit inside the weekend?

Five tasks in a shutdown window where every estimate has been more than doubled. The two buffer methods disagree by seven hours, and this time the one that runs leaner on long plans is the cautious one.

Words on this sheet

  • Predecessors: The tasks that have to finish before this one can start. Put their IDs here, separated by commas, and leave it blank when nothing has to come first.
  • F statistic: The variation between the groups divided by the variation inside them.

Operations Advanced Critical Chain Pro engine

Install free (opens the Google Workspace Marketplace in a new tab) How critical chain planning works

After you install, this is the model to open.

How Much Buffer Does the Changeover Need?

  1. In your spreadsheet, click the Sortia icon in the strip of icons down the right-hand edge. No strip? Click the arrow at the bottom-right to open it. You can also use Extensions, then Sortia, then Open Sortia.
  2. Click Start from a template and put that name in the search box.
  3. Pick the card with that name and click Load this template. It arrives on a new tab with real numbers already in it.

This one runs on a Pro engine, and every free install includes five full-quality runs on your own numbers, shared across all five Pro engines rather than five for each. After that, Pro is $199/year.

The answer

The plan on paper
106 hours of safe estimates against a 72-hour window
The critical chain
40 hours of focused work, lock-out through sign-off
Commit to
66.9 hours: the chain plus one 26.9-hour project buffer
Left in the window
5 hours, pooled at the end where you can watch it burn

A production line changeover inside a 72-hour weekend window, measured in hours rather than days. Every task carries two numbers: the focused duration, which is what the job takes when the spares are on the trolley and nothing has seized, and the safe duration, which is what maintenance wrote on the plan. The gap is enormous. Focused work totals 45 hours and the safe estimates total 106, which is more than a doubling on five of the six lines, and 106 hours does not fit inside a 72-hour window at all.

That is the situation this template starts from, and it is why the plan currently says the line restarts on Tuesday. Click Run with RSEM selected. The chain is the lock-out, the strip, the tooling install, the commissioning and the sign-off, 40 hours long on focused durations. The project buffer is 26.9 hours and the commitment is 66.9, so the line restarts inside the window with about five hours to spare, and the safety now sits in one place at the end where you can watch it burn instead of being hidden inside five separate estimates where it will be spent whatever happens.

The pre-kitting job feeds the tooling install and gets a feeding buffer of its own, 4 hours. Now switch Buffer sizing to the 50% rule and run it again, because the comparison is the reason this template exists. The 50% rule puts the project buffer at half the chain, 20 hours, for a 60-hour commitment, nearly seven hours leaner than RSEM. That is the opposite of what the shipped certification-chain template produces, where RSEM returns 17.9 days against the 50% rule's 25.5 and is the leaner of the two.

Both results are correct and the reason is in the arithmetic. The 50% rule grows with the length of the chain and knows nothing about the safe estimates at all. RSEM is the square root of the summed squared padding, so it grows with how much safety was hidden. On a long chain with moderate padding, chain length dominates and RSEM comes out smaller.

On a short chain carrying a doubling on every task, the padding dominates and RSEM is the larger, more cautious number. So choosing between them is not a question about how conservative you feel. It is a question about whether you trust the safe estimates. If those padded numbers came from people who know the job, RSEM is using real information and is the right answer here.

If they came from a spreadsheet that added forty percent to everything, they carry no information and the 50% rule is the honest fallback. One limitation to hold on to: the tool finds the chain as the longest path on focused durations and does not level resources, so if the same fitting crew is on the tooling install and the pre-kitting at the same time, this plan is optimistic and only you can see that.

Second thing to try: cut the safe duration on the commissioning step from 28 hours to 18, which is what a rehearsed first-article procedure buys you, and run again on RSEM. The project buffer falls to 22.4 hours and the commitment to 62.4. You removed hidden safety rather than removing work, and the plan got four and a half hours shorter without anybody moving faster.

To adapt it, put your own changeover steps in, get a focused number and a safe number for each in the same conversation, and hold the buffer at the end of the window as one number the shift leader owns.

The model

It arrives on a tab called Template: Line Changeover, carrying these columns:

  • ID
  • Task
  • Predecessors
  • Focused duration (hours)
  • Safe duration (hours)

Once it is in your sheet

  1. The model arrives with real numbers in it and runs as it stands, so you can press the button first and understand it second.
  2. Change the numbers to yours. The sheet marks which cells are inputs and which hold formulas, and most labels carry a note explaining the row.
  3. Press the run button at the bottom of the panel. It is labeled for the tool you are in, and the result lands on its own tab, with a written reading of it beside the figures.

Never used Google Sheets? Start here goes the whole way, in seven steps, and assumes nothing.

Next question

Every model like this one, and the method behind them: Critical chain planning.

Install free (opens the Google Workspace Marketplace in a new tab) All 379 templates