"IDK how long it will take"

I followed all the right steps. Experts shared their views that my project plan was workable. The people doing the work agreed it was good. The resources were available... and yet, somehow... the longest duration task ran 50% longer than planned.

What went wrong? Insufficient work breakdown ("chunking") and poor measurement.

I learned my lesson. On the next project facing the same large task, I broke it down into smaller sections. I assigned an owner to each section and got all the same input and advice to ensure it was doable. I made something like a burndown chart and I scheduled weekly standup's to review progress. This time, the total activity ran even longer. Late again.

What went wrong? The smaller sections of work were iterative and never wrapped-up they way I expected; loose ends were everywhere and came back.

I learned my lesson. On the next project, I engaged by subject matter experts more heavily to review the quality of work and ensure the technical debt was reasonable. I asked them to subjectively assess the "pace of work" for each section, rather than driving towards specific deadlines. The result? Closer to on time, but still late.

What went wrong? I'm not sure. A dozen issues together, perhaps. Team friction, confusion, unnecessary rework, etc.

I learned my lesson. Some things just can't be measured or predicted well. There are too many variables, too many inputs, too many options.

In these situations, it's better to work at the fringes of the problem just much as you hit it directly. For example:

• Start the task early
• Get more, better resources (but not too many - that causes other problems)
• Assume chaos and delay are the natural outcome
• Assume more time for rework will be needed
• Discourage optimization - focus on 'generally correct' until the every end

This isn't a great solution. But sometimes you just don't know how something will go so you can't predict it's duration. Just make the next best choice and maintain a sense of urgency.