Solving Percentage Errors: Different Magnitudes for Positive & Negative

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Discussion Overview

The discussion revolves around the concept of percentage errors in measurements, particularly focusing on why positive and negative errors can exhibit different magnitudes when calculating the area of an annulus from two circular areas. The scope includes mathematical reasoning and conceptual clarification regarding error propagation in measurements.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant questions how positive and negative errors can differ in magnitude when calculating the area of an annulus.
  • Another participant suggests that the difference arises from varying denominators in percentage calculations, using a money analogy to illustrate the concept.
  • A different participant expresses confusion regarding the application of the money analogy to area calculations, seeking clarification.
  • One participant provides an example involving a square with sides measured with a precision of 10%, demonstrating how the maximal deviations in area can differ based on whether the sides are longer or shorter.
  • Another participant emphasizes that the error in area calculations is not symmetric due to the nature of area being proportional to the square of the length, leading to asymmetric uncertainties.
  • There is a clarification that the observation of asymmetric uncertainties is not necessarily framed as a problem, but rather a characteristic of the measurement process.

Areas of Agreement / Disagreement

Participants express differing views on the implications of asymmetric errors, with some focusing on the mathematical reasoning behind it while others seek clarification on its relevance. The discussion does not reach a consensus on whether this asymmetry is problematic.

Contextual Notes

The discussion highlights the dependence of error magnitudes on the method of measurement and the mathematical operations performed, but does not resolve the underlying assumptions or implications of these observations.

lavster
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How can you get positive and negative errors to be different in magnitude?

for example -

when calculating the error of the area of an annulus from two circular areas (ie subtracting one from the other, why is the positive error greater than the negative error

Thanks
 
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because the denominators are different...

like, say, you have $100 invested and you lose $20...that's a $20 loss...
Now you have $80...What percentage gain do you need to get your $20 back...
20/80 is 25%.

figures don't lie, but liars figure!
 
i understand the money analogy, but not when talking about the areas - sorry! :S we are only subtracting once
 
lavster said:
i understand the money analogy, but not when talking about the areas - sorry! :S we are only subtracting once

Could you illustrate by an example what you are concerned about?
 
Imagine a square where both sides are known with a precision of 10% - they might be 10% shorter or 10% longer, but not more. What is the maximal deviation?

Larger area: Both sides 10% longer, total area 1.1^2 = 1.21 of the original area (21% more).
Smaller area: Both sides 10% shorter, total area 0.9^2 = 0.81 of the original area (19% less).
Do you see the difference?
 
mfb said:
Imagine a square where both sides are known with a precision of 10% - they might be 10% shorter or 10% longer, but not more. What is the maximal deviation?

Larger area: Both sides 10% longer, total area 1.1^2 = 1.21 of the original area (21% more).
Smaller area: Both sides 10% shorter, total area 0.9^2 = 0.81 of the original area (19% less).
Do you see the difference?

I see the difference, but why do you see this as a problem?
 
The error depends on how the measurement was taken and what analysis was done. When you do math on the measurement, the errors will change shape. If you measure a circle's radius with a ruler with a symmetric error in the length, then the error in the area is not symmetric, because area goes as length squared.
 
mathman said:
I see the difference, but why do you see this as a problem?
Where did I say that it is a problem?
I just said that you can get asymmetric uncertainties in this way.
 
ah great :) thanks :)
 

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