# Understanding a simple math example about Doubly Robust method of moments

I want help understanding this example on doubly robust moment conditions.

No following text helps explain this example as it is independent of the earlier text.

The things I don't understand is the meaning of are: In the small psi equation right in the middle of the example (the equation standing free of text in the middel), I would be glad to know what every thing is and why a "small $$d$$" is used, why divide with "$$\pi$$", why add $$\mu(x)$$ and why subtract $$\theta$$. Can anybody tell me why small $$\psi$$ looks as it does and where in the world that "formula" come from. It looks like a $$z$$-value buy I dont know! $$???\quad\frac{d(y-\mu(x))}{\pi(x)} + \mu(x) - \theta\quad???$$

Example 1