# Directly discarding big term in the proof of error propagation formula of variance from random variable $x$ to $f(x)$?

I read the error propagation formula scanario said that, the connection between the variance of a random variable $$x$$ and $$f(x)$$ is $$\frac{var(f(x))}{|\partial_xf(x)|_{x=\bar{x}}|^2}=var(x)$$. While I'm confused in the prove of it in the process of Taylor series:

The proof of it states that, if we define the expected value of random variable $$x$$ and variance of it as follows: \begin{aligned} \bar{x} &=\sum_{i=1}^{N} p_{i} x_{i} \\ (\Delta x)^{2} &=\sum_{i=1}^{N} p_{i}\left(x_{i}-\bar{x}\right)^{2} \\ &=\overline{x^{2}}-\bar{x}^{2} \end{aligned} And the same for $$f(x)$$: \begin{aligned} \overline{f(x)} &=\sum_{i=1}^{N} p_{i} f\left(x_{i}\right) \\ (\Delta f)^{2} &=\sum_{i=1}^{N} p_{i}\left(f\left(x_{i}\right)-\bar{f}\right)^{2} \end{aligned}\tag{1} Then we can Taylor expanse it as: \begin{aligned} f\left(x_{i}\right) &=\left.\sum_{n !}^{\infty} \frac{1}{n !}\left(x_{i}-\bar{x}\right)^{n} f^{(n)}(x)\right|_{x=\bar{x}} \\ &\approx f(\bar{x})+\left(x_{i}-\bar{x}\right) f^{(1)}(\bar{x}) \end{aligned}\tag{2} ignoring the square and higher-order term. Then $$\overline{f(x)}$$ can be written as \begin{aligned} \overline{f(x)} &\approx f(\bar{x})+\overline{\left(x_{i}-\bar{x}\right)} f^{(1)}(\bar{x}) \\ &=f(\bar{x}) \end{aligned}\tag{3} Then replace eq (3) and (2) into (1)， we can get $$\frac{var(f(x))}{|\partial_xf(x)|_{x=\bar{x}}|^2}=var(x)$$.

My question is, $$x_i-\bar{x}$$ is not always a small term!!! Why should we just discard it? Or, are there more formal references of this error propagation formula, because all I search online is some other kind of form.

• 1) Where did you read that proof? 2) If an overbar means "expected value", note that you don't have $x_i -\bar{x}$ in equation 3, you have $\overline{x_i-\bar{x}}$ in equation 3, and the expected value of $x_i - \bar{x} = 0$. Oct 15, 2021 at 1:50
• @jbowman The proof comes from a Ph.D. thesis in my language. And I've amended something in eq(3). As for the overbar thing in your comment, yes, $x_i-\bar{x}=0$(just add the whole step to make the process more transparent), which makes it into the final form of eq(3). Thanks! Oct 15, 2021 at 5:34
• I've taken the liberty of replacing a few incorrect $=$ with $\approx$. Oct 16, 2021 at 14:39

My question is, $$x_i-\bar{x}$$ is not always a small term!!! Why should we just discard it? Or, are there more formal references of this error propagation formula, because all I search online is some other kind of form.

Indeed $$x_i-\bar{x}$$ is not always small but the average of it $$\overline{x_i-\bar{x}}$$ is zero. You could write is as $$\overline{x_i-\bar{x}} = \overline{x_i} - \overline{\bar{x}} = \bar{x_i} - \bar{x}=0$$

Sidenote: when you apply this linear approximation then $$x_i-\bar{x}$$ should actually be small. In this question about a log transformation, you see how the linear approximation works when the deviation are relatively 'small'. (It is also shown how it does not work and how a better approximation can be made)

In that question the following image was used, to show that a normal distribution (or something that looks like a normal distribution) becomes nearly linear transformed with a log transformation, when the deviations are not large.

You can see this similar for the error propagation. With the linear approximation we approximate the curve of the function by a straight line and we use the slope of the function to compute how the deviations changing with an approximately constant factor

The computation like it is shown above discards terms which have higher order than 1 when the function is expanded. Therefore, the proposed formula only applies to functions f which are ("well enough") representable as a linear function. This is the level of error propagation like you will typically find it.

If you want to do error propagation beyond linear taylor expansion because your function f is highly non-linear (or to check the assumptions made during propagation of error) I suggest to go back to the idea of error propagation itself. The following image shows the limitation of using a linearization (resulting in $$+/- \Delta y$$) instead of all terms of the taylor expansion (resulting in $$+\Delta y_+$$ and $$- \Delta y_-$$):

Biggerj1, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons