Carry uncertainties through each calculation using propagation rules and report the final result rounded to the same decimal place as the combined uncertainty.
When you propagate uncertainty, treat each number you calculate as having a best value and a wiggle room around that value. For adding or subtracting, add the wiggle rooms in quadrature by taking the square root of the sum of their squares, and for multiplying or dividing, add relative wiggle rooms in quadrature. If you raise a number to a power, multiply the relative wiggle room by the absolute value of the power. Work with the same units and keep track of significant figures so the uncertainty stays meaningful, and only round the final value once to match the precision of the final uncertainty.
Students often just round every intermediate number which hides how errors grow, so keep extra digits until the very end and then round the final answer to match the uncertainty. Another common mistake is treating uncertainties like fixed plus or minus numbers when they are relative; convert to relative uncertainty for multiplication and division and then convert back at the end. Some people forget to combine systematic and random components; estimate and combine them appropriately and include both in the final uncertainty.
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Propagating Uncertainty Through Calculations
- Treat numbers as value plus uncertainty
- Add uncertainties in quadrature for sums
- Add relative uncertainties in quadrature for products
- Multiply relative uncertainty by power for exponents
- Keep consistent units throughout calculations
- Preserve extra digits until final rounding
Common Mistakes in Reporting Uncertainty
- Rounding intermediate results too early
- Treating uncertainties as fixed absolute values
- Forgetting to combine systematic and random components
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