Realistic estimate
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After known biases
Pessimistic (safe)
—
For scheduling purposes
What you're probably underestimating
What will actually slow you down
One genuinely useful thing to do
Methodology: Multipliers are based on well-documented cognitive biases: the planning fallacy (people underestimate task duration by 25–50% on average), optimism bias (we expect better-than-average outcomes), and scope creep (tasks expand). Realistic estimate applies a 1.5–2.5x multiplier by task type; pessimistic applies an additional 1.5–2x buffer for things that go wrong. These are starting points, not guarantees. The part where you know your own situation is important.