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I needed some help trying to understand why the sum of the observed values $Y_i$ equals the sum of the estimated values $\hat{Y}_i$.

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Let us consider the special case of a simple linear regression.

Minimizing the sum of squared residuals $S=\sum_{r=1}^{n}(y_{r}-a-b x_{r})^{2}$ w.r.t. $a$ and $b$ gives the "normal equations" (first order conditions) $$ \sum_{r=1}^{n}y_{r}=na+b\sum_{r=1}^{n}x_{r}\quad\text{and}\quad\sum_{r=1}^{n}y_{r}x_{r}=a\sum_{r=1}^{n}x_{r}+b\sum_{r=1}^{n}x_{r}^{2}. $$ The fitted values are $\hat{y}_{r}=a+bx_{r}$, with residuals $$\hat{u}_{r}=y_{r}-(a+bx_{r})$$ Consider $$\sum_{r=1}^{n}\hat{u}_{r}=\sum_{r=1}^{n}y_{r}-\sum_{r=1}^{n}\hat{y}_{r}=\sum_{r=1}^{n}y_{r}-\sum_{r=1}^{n}(a+bx_{r})=\sum_{r=1}^{n}y_{r}-na-b\sum_{r=1}^{n}x_{r}.$$ Now, substitute $\sum_{r=1}^{n}y_{r}$ for the first normal equation.

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Let $P$ be the projection matrix on $X$, where one of the columns of $X$ is $\mathbf{1}$, where $\mathbf{1}$ is vector of ones, then \begin{align} \mathbf{1}'\hat{y}&=\mathbf{1}'Py\\ &=\left(P'\mathbf{1}\right)'y\\ &=\mathbf{1}'y. \end{align}

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