Uncertainty of experiment relative or absolute

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Discussion Overview

The discussion revolves around the concepts of relative and absolute uncertainty in experimental measurements, specifically in the context of calculating uncertainties based on time measurements and their inverses. Participants explore how to distinguish between these two types of uncertainties and provide examples to illustrate their points.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant calculated uncertainty as a percentage of their results and seeks clarification on whether this represents relative or absolute uncertainty.
  • Some participants propose that the uncertainty calculated as a percentage indicates relative uncertainty.
  • Another participant requests further clarification on how to determine if uncertainty is relative or absolute.
  • It is suggested that relative uncertainty is indicated when expressed as a percentage, while absolute uncertainty is when the error is quoted in the same units as the measurement.
  • A participant provides a method to distinguish between the two types of uncertainty, stating that if the uncertainty has units, it is absolute; if it is dimensionless, it is relative.
  • Another participant emphasizes that the context of the measurement affects which type of uncertainty is more relevant, citing examples involving digital chronometers and their potential errors over time.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of relative and absolute uncertainty, but there is some variation in how these concepts are applied in specific contexts. The discussion remains somewhat unresolved as participants seek further clarification and examples.

Contextual Notes

Some assumptions about the definitions of relative and absolute uncertainty may not be explicitly stated, and the application of these concepts can depend on specific experimental conditions and contexts.

hulk78
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I calulated the time(s), then i found the uncertainty as a percentage of my results. Later on i calculated 1/time and used the uncertainty % which i originally calulated.

Could somebody tell me if it is relative or absoulte uncertainty and why?
 
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It appears (as best as I can understand what you did) to be relative uncertainty.
 
mathman said:
It appears (as best as I can understand what you did) to be relative uncertainty.

But could you please tell me how to know if the uncertainty is relative or absolute?
 
It's relative because you calculated the uncertainty as a percentage of your results. It's absolute if you quote the error in the same units as the measurement.

I expect this would mean something like "10 seconds with 10% error" i.e. 10 +/- 1 s (absolute)

The result of inversion is "0.1 Hz with 10% error", i.e. 0.1 +/- 0.01 Hz (absolute)
 
A quick and dirty way to understand what you ask is to see if the uncertainty has units. If yes it is absolute (e.g in Mikey's example
10+/- 1 sec , the error=1 is measured in seconds=>absolute).
Else, if you measure the error/mean this is the relative uncertainty -> it is dimensionless
 
I can understand that you don't want to get it 'wrong' but, once you see the logic of the distinction, I think you will be able to use the terms appropriately.

If your uncertainty is expressed as a "percentage" then it is Relative, by definition and, if it is given in actual units, it would be absolute.
A digital chronometer can be a couple of seconds wrong over a month. Those two figures, taken together, tell you both the relative and absolute uncertainty and which you use will depend on that actual application. You can be pretty sure that the time will not drift by more than a small fraction of a second over one hour and that might be very relevant for some measurements - the fact that it could be two seconds out would not matter if you are measuring someone's lap time.
 

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