[Cthulhu_]

Exactly; you can only provide a more accurate estimation if you know all the details and the exact implementation and how fast you can type, and even there you have to take at least a 30% margin either way.

You can only be 100% accurate when it's done. Here's a chart: https://cdn-images-1.medium.com/max/1600/1*hS2GObQarMYhamPeV.... Basically, the closer you are to done, the more accurate your estimation will be.

[amthewiz]

To go even further on that point - the chart itself seems overly optimistic. +-40% of estimate after product design? +-10% of estimate after design spec?! Countless number of times I have seen detailed design specs completely thrown out of whack by new engineering, process or business revelations/insights. Human thought is iterative - most people can imagine at most 80% of the future/product/design possibilities.