# Where does Jensen's Inequality come into the EM derivation?

I am working my way through the original EM paper Maximum Likelihood from Incomplete Data by Dempster, et al.

I have run into a problem with a statement made in section 3. "General Properties". Specifically I am having a difficult time with Lemma 1:

My understanding of Jensen's inequality is

$$\mathop{E}[f(X)] \geq f(\mathop{E}[X]) \text{ for convex functions } f(x)$$

or the reverse for concave functions (e.g. the log of a pdf).

If I am interpreting equation $$3.3$$ correctly, we have

$$\mathop{E}[\log k(x|y, \phi')|y, \phi] \leq \mathop{E}[\log k(x|y, \phi)|y, \phi] \\ \implies \int_X \log k(x|y, \phi') p(x | y, \phi) dx \leq \int_X \log k(x|y, \phi) p(x | y, \phi) dx$$

I am failing to see where Jensen's inequality is applied as both sides of the equation include the function $$\log k(\cdot)$$ inside of the expectation.

Am I misunderstanding the application of Jensen's inequality?

\begin{align} H(\phi' \mid \phi) - H(\phi \mid \phi) & = E[\log k(x \mid y, \phi') \mid y, \phi] - E[\log k(x \mid y, \phi) \mid y, \phi] \\ & = E[\log \{k(x \mid y, \phi') / k(x \mid y, \phi) \} \mid y, \phi] \\ & \leq \log \{ E[k(x \mid y, \phi') / k(x \mid y, \phi) \mid y, \phi] \} \\ & = \log \int_{\mathcal{X}(y)} k(x \mid y, \phi') dx \\ & = 0 \end{align}