expected value of a score function (the gradient of the log-likelihood function) according to the Wikipedia: https://en.wikipedia.org/wiki/Score_(statistics), expected value of a score function should equals to zero and the proof is following:
\begin{equation}
\begin{aligned}
\mathbb{E}\left\{ \frac{ \partial }{ \partial \beta }  \ln \mathcal{L}(\beta|X) \right\} &=\int^{\infty}_{-\infty} \frac{\frac{ \partial }{ \partial \beta } p(X|\beta)}{p(X|\beta)} p(X|\beta) dX \\
&= \frac{ \partial }{ \partial \beta }\int^{\infty}_{-\infty} p(X|\beta) dX  =  \frac{ \partial }{ \partial \beta } 100\% = 0
\end{aligned}
\end{equation}
My question is why the probability density function of random variable $\frac{ \partial }{ \partial \beta }  \ln p(X|\beta)$ is $p(X|\beta)$? Many thanks!!
 A: Method 1


*

*You apply:
a rule for the derivative of a logarithm
$$\frac{\partial}{\partial y} \log\left[f(x,y)\right] =
   \frac{\frac{\partial}{\partial y} f(x,y) }{f(x,y)}$$

*And you apply: 
that the expectation of any value derived from the data $T(X)$
$$E\left[T(X) \right] = \int_{-\infty}^\infty \overbrace{T(x)}^{\llap{\text{value of}}\rlap{\text{ $T(X)$ at point $x$}}} \underbrace{f(x)}_{\llap{\text{density of}}\rlap{\text{ r.v. $X$ at point $x$}}} dx$$ 
for example, you may be used for the equations of the n-th moment of $X$
$$E\left[X^n \right] = \int_{-\infty}^\infty x^nf(x) dx$$ 
So combining those two together:
$$\begin{array}{}
E\left[\frac{\partial}{\partial \beta} \log\left[f(x,\beta)\right] \right] &=& E\left[ \frac{\frac{\partial}{\partial \beta} f(x,\beta) }{f(x,\beta)} \right]\\
&=&  \int_{-\infty}^\infty \frac{\frac{\partial}{\partial \beta} f(x,\beta) }{f(x,\beta)} f(x,\beta) dx \\
&=&  \int_{-\infty}^\infty \frac{\partial}{\partial \beta} f(x,\beta)  dx \\
&=& \frac{\partial}{\partial \beta} \int_{-\infty}^\infty  f(x,\beta)  dx \\
&=& \frac{\partial}{\partial \beta} 1 \\
&=& 0
\end{array}$$
Method 2
And this relates to your question


*

*You apply the transformation rule for a random variable say $g(t)$ is the distribution for $T$ and $f(x)$ is the distribution of $X$, then if $x=h(t)$ you transform from $f(x)$ to $g(t)$ by using
$$g(t) = f(h(t)) \left| \frac{\partial h(t)}{\partial t} \right|$$
So the distribution function $g(t)$ of  $T = \frac{\partial}{\partial y} \log\left[f(x,y)\right]$ is as following:
$$ g(t) = \text{I am here a bit stuck to express this but let's continue} $$
(it is not so easy)
And the expectation will be computed as:
$$E\left[ \frac{\partial}{\partial y} \log\left[f(x,y)\right]  \right] = E\left[t \right] = \int_{-\infty}^{\infty} t g(t) dt$$
So with this second method you differentiate with $dt$ and not $dX$. And the derivation on Wikipedia is using method 1, where you should not interpret part of the integrand as the density of $T$ but as the density of $x$. You use $E\left[t \right] = \int_{-\infty}^{\infty} t g(t) dt$ or you use  $E\left[t \right] = \int_{-\infty}^{\infty} t(x) f(x) dx$ 
A: Because your random variable is still $X$ with pdf $p(X|\beta)$.
When you take the expectation with respect to $X$ of $\frac{ \partial }{ \partial \beta }  \ln \mathcal{L}(X|\beta)$ this is just a function of $X$, hence the pdf is $p(X|\beta)$.
I see it's also called "law of the unconscious statistician".
