# Uncertainty: Systematic & Random

• ChiralSuperfields
In summary, systematic uncertainty is the total amount by which the measured value may differ from the actual value, while random uncertainty is the amount by which the measured value may vary from one observation to another. Both may be (consistently) interpreted as fractional or absolute.
ChiralSuperfields
Homework Statement
Dose anybody please know what the relationship between absolute uncertainty, systematic uncertainty and random uncertainty is?
Relevant Equations
Equation above.
I am thinking that it might could be absolute uncertainty = systematic uncertainty + random uncertainty.

Many thanks!

Any particular reason ?

ChiralSuperfields
As far as I am aware, absolute uncertainty means the absolute amount by which the measured value may differ from the actual value. This is as opposed to fractional uncertainty, which is absolute uncertainty divided by the measured value.
And I find "systematic uncertainty" conceptually awkward. Systematic error is the more usual expression.
So I would say that total error is systematic + random, where each of those may be (consistently) interpreted as fractional or absolute.

That is with regard to repeated measurements which are in principle of the same quantity. If they are for different quantities (because some parameter is being varied) these errors may vary in different ways. E.g. the systematic fractional error my remain constant, while for random error it is the absolute error that is constant.

Last edited:
ChiralSuperfields
BvU said:
Any particular reason ?
haruspex said:
As far as I am aware, absolute uncertainty means the absolute amount by which the measured value may differ from the actual value. This is as opposed to fractional uncertainty, which is absolute uncertainty divided by the measured value.
And I find "systematic uncertainty" conceptually awkward. Systematic error is the more usual expression.
So I would say that total error is systematic + random, where each of those may be (consistently) interpreted as fractional or absolute.

That is with regard to repeated measurements which are in principle of the same quantity. If they are for different quantities (because some parameter is being varied) these errors may vary in different ways. E.g. the systematic fractional error my remain constant, while for random error it is the absolute error that is constant.
Thank you for your replies @BvU and @haruspex!

@BvU I can't remember now sorry.

@haruspex thank you that helps

Many thanks!

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