# Where does the delta method's name come from?

Where does the delta method's name come from?

I don't see anything related to "$\epsilon$-$\delta$", nor Dirac delta for example.

• merriam-webster.com/dictionary/delta – whuber Feb 25 '17 at 20:25
• Delta-method is closely related (sometimes synonym) to the concept of propagation of error, e.g. see this. The name follows probably from the fact that $\delta$ and $\Delta$ are used to denote error in calculus. – Francis Feb 26 '17 at 3:37
• @Francis So does it come from Taylor series? Since it has that error term? – mavavilj Feb 26 '17 at 5:45
• @mavavilj: could be, but I haven't found a source to confirm this. – Francis Feb 26 '17 at 17:01
• @Francis Indeed, Taylor series, see my answer edit. – Carl Mar 30 '18 at 1:22

The name "Delta" is from the symbol $\Delta$ for "change" which is used in limit expressions like let $\Delta X_i\rightarrow 0$, where $\Delta X_i=X_{i+1}-X_i$, and also $\Delta$ or lower case "δ" refers to an inexact, non-zero differential equation (i.e., before limits are taken) which reduces in the limits to a differential equation the latter using infinitesimals. For example, $\Delta f(x)= f(x+\Delta x)-f(x)$, which reduces to $\partial f$, a vanishingly small differential, for $\Delta x$ sufficiently small. Now sometimes we want to manipulate equations so that we are not dividing or multiplying by zeros, and in that case, we might use the $\Delta$ notation.
The name delta method, is probably only used by statisticians. In other contexts, e.g., engineering, physics, it would be more commonly referred to as propagation of error. To see how the error propagation includes the $\Delta$ symbol, see propagation of error.
 Edit

Error propagation is from Taylor series, useful examples. Indeed, when extended to the first two derivatives of Taylor series, i.e., $\mathbf{Var}[Y]\approx(g'(\mu_X))^2\sigma_{X}^{2}+g'(\mu_X)g''(\mu_X)\mu_3+\frac{1}{4}(g''(\mu_X))^2(\mu_4-\sigma_{X}^{4})$, it becomes more accurate, and I would venture that that is needed when the solution space curvature is of greater magnitude.
BTW, in ancient Greek the letter $\Delta$ was pronounced as a "d" sound and in modern Greek this is a hybrid "d" and "th" sound. The etymology is purported to be as follows,