# How is to maximize a function $f(x,y)$ for values of $y$?

### Maximum Likelihood Method:

Likelihood function asks what value of parameter, $$\theta$$, makes the data set most probable.

Let the distribution is

$$f(x;p)=\binom{3}{x}p^x(1-p)^{n-x},\quad x=0,1,2,3.$$

Again, suppose the parameter $$p$$ can assume one of the two values $$\frac{3}{4}$$ or $$\frac{1}{4}$$.

The possible outcomes and their probabilities are given below:

$$\begin{array}{c|ccc} x & 0 & 1 & 2 & 3 \\ \hline f(x;\frac{3}{4}) & \frac{1}{64} & \frac{9}{64} & \frac{27}{64} & \frac{27}{64} \\ f(x;\frac{1}{4}) & \frac{27}{64} & \frac{27}{64} & \frac{9}{64} & \frac{1}{64} \end{array}$$

The maximum likelihood estimator may be defined as

$$\hat p=\hat p(x) = \begin{cases} .25, & \text{for x=0,1} \\[2ex] .75, & \text{for x=2,3}. \end{cases}$$

### Profile Likelihood Method:

Suppose we have two parameters, $$\theta$$ and $$\delta$$, where $$\theta$$ is of interest and $$\delta$$ is a nuisance parameter.

The profile likelihood of $$\theta$$ is

$$L_p(\theta)=\max_\delta L(\theta,\delta),$$

where $$L(\theta,\delta)$$ is the "complete likelihood".

$$\max_\delta L(\theta,\delta)$$ expresses maximize the likelihood function of $$\theta$$ and $$\delta$$, $$L(\theta,\delta)$$, over $$\delta$$ values.

### My question is:

How is to maximize a function over values of a variable? That is, how is to maximize a function $$f(x,y)$$ for values of $$y$$?

## 1 Answer

Continuous parameters can be handled with differentiation and analysis. If the parameters are discrete as in your first example, you first fix $$\theta$$ and $$x$$, then for all $$\delta$$, you'll search for maximum values of the likelihood, and finally you'll obtain a discrete function $$L_p(\theta,x)$$ as a table.