mean, and standard deviation question.

[faust9]

Here's the question:

In the expression for uniaxial strain e = d/l, the elongation is specified as
d ~ N(0.0015, 0.000092) in. and the length l ~ N(2.0000, 0.0081) in.
What are the mean, standard deviation, and coefficient of variation of the corresponding strain e?

so, I said the mean (X) of delta is 0.0015 and the standard deviation (S) of delta is 0.000092

X_d=0.0015, S_d=0.000092
X_l=2.000, S_l=0.0081

I said Z=d/l\ thus\ X_z=X_d/X_l and the S_z^2=(C_d^2+C_l^2)/X_z^2


So, I did the following: X_e=X_d/X_l=0.0015/2.000=0.00075

and S_z=\sqrt{((0.000092/0.0015)^2+(0.0081/2.000)^2)/0.00075^2}=81.956

The last part doesn't seem right though. Any thoughts?

Thanks.

[ehild]

Here's the question:

In the expression for uniaxial strain e = d/l, the elongation is specified as
d ~ N(0.0015, 0.000092) in. and the length l ~ N(2.0000, 0.0081) in.
What are the mean, standard deviation, and coefficient of variation of the corresponding strain e?


X_d=0.0015, S_d=0.000092
X_l=2.000, S_l=0.0081

I said Z=d/l\ thus\ X_z=X_d/X_l and the S_z^2=(C_d^2+C_l^2)/X_z^2



If you denoted the relative errors by C

S_z^2=(C_d^2+C_l^2)*X_z^2 instead of what you used.