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I am relearning econometrics to get a better understanding of it, and to clear the confusions when I had in college.

Using the simple regression model, we have a population model equation as:

$$ y = beta_{0} + beta_{1}x + utag{1}$$

In the SLR assumption 3, we have the zero conditional mean assumption. Are we assuming this statement because in reality, y can take many values given x taking a single value, so that we hope, given x, the expected value of y is center around E[y|x], is this understanding of SLR Assumption 3 correct? This means, if we take the expected value of equation (1) conditioned on x:

$$ E[y|x] = beta_{0} + beta_{1}x + E[u|x]tag{2}$$

Because any deviation can be absorbed by the intercept item, we lose nothing by assuming E[u] = 0. By assuming SLR 3 E[u|x] = E[u] = 0, we are implying that:

$$ E[u|x] =sum{}u P_{u|x}(u) = sum{}udfrac{P_{u,x}(u,x)}{P_{x}(x)}=sum{}udfrac{P_{u}(u)P_{x}(x)}{P_{x}(x)} = sum{}uP_{u}(u) = E[u]tag{3}$$

In order to get the above equation, we are implying that x and u are independent of each other, so we are also implying that from the covariance formula of x and u, we can get the following equation E[ux] = E[u] = 0:

$$ Cov(u, x) = E[ux] - E[u]E[x] = E[ux] - 0 = E[u]E[x] = 0 tag{4}$$

Thus:
$$ E[ux] = 0 tag{5}$$

And, because of this implied uncorrelated relationship between u and x, the equation (2) above can be viewed as when E[u|x] = 0, so we have the population regression function by taking expectation conditioned on x for equation (1), as:

$$ E[y|x] = beta_{0} + beta_{1}x tag{6}$$

This is a linear relationship between x and expected value of y, by the change of 1 unit in x leads to beta1 unit change in y. And the distribution of y is centered at E[y|x].

So my question is that, when we are estimating using OLS, is the sample regression function estimating the population model equation (1) or estimating the population regression function equation (6) and why?

Also, in multiple regression function, we also assume zero conditional mean as:
$$ E[u|x_{1}, x_{2}, x_{3},...,x_{k}] = 0 $$
Here are we saying that u is uncorrelated with the group of (x1,...xk), or can we say that u is uncorrelated with each of xi respectively, for i = 1,...,k?

Thank you for your help and time! Much obliged.

The OLS regression estimates the conditional expectation, i.e.,
$$
mathbb{E}[y|X=x]=beta_0 + beta_1x,
$$
namely, the estimated model is
$$
widehat{mathbb{E}[y|X=x]}=hat{y} = hat{beta_0}+hat{beta}_1x.
$$
There is no sense in estimating $u$ as $u$ is a random variable.

Now, let's see what happens if
$
mathbb{E}[u|x]
$
is not zero. If it constant then you stated correctly that it can be absorbed into the intercept term. Another possibility is that it depends somehow on $X$, i.e.,
$$
mathbb{E}[u|X=x] = g(x),
$$
so the conditional expectation is
$$
mathbb{E}[y|X=x]=beta_0 + beta_1x + g(x),
$$
now everything depends on the structure of $g$. If it is linear, then you go back to the original model
$$
mathbb{E}[y|X=x]=beta_0 + beta_1x + bx = beta_0+(beta_1+b)x=beta_0+tilde{beta}x.
$$
If it has any other parametric structure, then it modifies the model according to its structure, if $g$ is non-parametric or non-measurable then the linear simple model is simply inappropriate in this case. And regarding the independence, this is very strong assumption. All the basic assumptions assume that the variables are uncorrelated or ($y$s are) conditionally (on $x$) independent. It is enough for the Gauss-Markov theorem to apply.