Biggest challengues building and estimating project schedules

I think schedule building is one of those things that looks simple on paper but gets tricky in practice. At least for me, the biggest challenges are usually:

Optimism bias in estimates, because usually teams tend to underestimate effort because they think in “happy path” terms. I’ve seen this especially in tasks involving new technology.
Also the hidden dependencies, you can map out tasks, but there are always those invisible cross-team dependencies that only surface once you’re mid-execution.
Something else I'm pretty sure we all have experienced a lot is the resource availability vs. reality: People are often allocated 100% on paper, but in reality they’re juggling multiple projects, support work, or even unplanned requests.
And finally changing scope mid-flight, I mean, even small scope shifts can ripple across the schedule, but they’re rarely factored into initial estimates.
So, some of the things that helped me here were:
- Running estimation workshops where multiple team members give input (planning poker, three-point estimation), I think it really balances optimism.
- Building buffers at key milestones (not just the end) so there’s flexibility without blowing up the whole plan.
- Using a rolling-wave approach, because you could plan high-level for the long term, but detail only the near-term phases where you have more certainty.

There are probably more things I've tried; nothing else comes to my mind, though, but at the end of the day, it’s a mix of discipline and humility: assume things will change, and build schedules that can absorb that.

Rami thanks for sharing these insights! It's definitely a challenge to build accurate schedules with so much uncertainty in the market.

I'm curious, how do you integrate these regular schedule updates and scenario planning with your change control process? In my company we only have 3 chances to update the schedule without penaly in the project grade!

Francisco.

I especially appreciate your point about the "happy path" bias. It's so true that teams often underestimate effort because they focus on the ideal scenario, ignoring potential roadblocks and hidden dependencies. This is a constant struggle! As well as your comments in 100% allocation and dependencies.

Your suggestions for mitigating these risks – estimation workshops, milestone buffers, and rolling-wave planning – are excellent strategies. Thanks again for sharing!

Hi Francisco Herrera, what a valuable initiative — and a crucial topic!

From my experience across both predictive and hybrid environments, the biggest challenges in building reliable schedules often fall into three interdependent traps:

- Over-optimism and underestimation — especially in early phases where scope is fluid and pressure is high.
Parkinson’s Law and the Planning Fallacy often sneak in unnoticed.

- Resource realism — not just availability, but actual productivity, multitasking overheads, and competing priorities.

- Invisible dependencies — especially cross-team or external ones that aren't visible in the WBS but can disrupt everything.

What helps?
- Progressive elaboration + rolling wave planning, especially when uncertainty is high.
- Three-point estimates (PERT) to expose variability — even if only informally.
- Historical data + post-mortems — past projects are goldmines of realism.
- Critical Chain thinking, to reduce multitasking and protect delivery dates with buffers.

And above all: collaborative scheduling.
When the team builds the plan with you — not just receives it — ownership, accuracy, and commitment increase dramatically.

Looking forward to learning from others’ insights as well!