## I need help with uncertainty of a curve fit

## I need help with uncertainty of a curve fit

(OP)

I'd like to know how I can find the uncertainty in "m" and "b", the two constants you get when you make a linear curve fit "y=mx+b".

I know there's a similar topic just below here

http://www.eng-tips.com/viewthread.cfm?qid=261269&page=1

But it seems to be that the mean and standard deviation obtained through that method has the same units of "y". How would I find uncertainties that have the same units as "m" and "b"?

For example, say I'm curve fitting stress and strain data to get the Young's modulus of a material. How would I determine the uncertainty in the Young's modulus, in units of MPa?

Thanks in advance.

I know there's a similar topic just below here

http://www

But it seems to be that the mean and standard deviation obtained through that method has the same units of "y". How would I find uncertainties that have the same units as "m" and "b"?

For example, say I'm curve fitting stress and strain data to get the Young's modulus of a material. How would I determine the uncertainty in the Young's modulus, in units of MPa?

Thanks in advance.

## RE: I need help with uncertainty of a curve fit

Consider the equation y = mx

You get an uncertainty (via comparison of data) for the modelled vs. actual y. The resulting "uncertainty" in m would then be simply calculated by U(m) = U(y)/x where U() are the uncertainties (errors).

In a similar manner, one could calculate uncertainties for both m and b, by plugging in the calculated value for m, and calculating what deviation in b is required to give the correct y value, and vice versa for m.

## RE: I need help with uncertainty of a curve fit

The textbook should then go further and show you how to use these quantities to estimate the variances associated with using your regression equation for predictions: V(y) and V(E(y)). Make sure you understand the subtle difference between these two.