# Log - likelihood function, why does the summation sign vanish?

I have the log-likelihood function:

$$l(p_i,y_i) = \sum_{i = 1}^n \left( \ln(p_i) + y_i \ln(1 - p_i) \right)$$

And I need to calculate the maximum likelihood estimator of $p_i$. When I do this, for some reason when differentiating, the summation sign vanishes. Why is this?

EDIT: To calculate the maximum likelihood estimators, I would differentiate my log likelihood, equal that to 0 and solve:

$$\frac{\partial{l}}{\partial{p_i}} = \sum_{i = 1}^n \left( \frac{1}{p_i} - \frac{y_i}{1 - p_i} \right) = 0$$

Rearranging gives me

$$\sum \frac{1}{p_i} = \sum \left( \frac{y_i}{1 - p_i} \right)$$

Oh, does this now become

$$\frac{n}{p_i} = \frac{n y_i}{ 1 - pi}$$

And then I can divide through by n and rearrange to get my value for $\hat{p_i}$?

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It would help if you showed the steps you are taking in calculating this. – Peter Flom Dec 25 '12 at 17:57
Say that you take the derivative with respect to $p_1$ The derivative "can enter the summation", by linearity. Now, $p_1$ appears in the first term ($i = 1$) only. The derivative of all other terms ($i \neq 1$) are thus $0$. This is why the summation sign disappears. – ocram Dec 25 '12 at 18:01
But in some of my other questions, I've had to leave the summation sign in to solve for my estimator. And would that not only give me the estimator for $p_1$ and not $p_i$? – Kaish Dec 25 '12 at 18:05
What I said is still valid for any $i$ between $1$ and $n$. $p_i$ appears in the $i$th term only; all other terms do not contribute to the derivative. Your first equation in your edit is thus wrong. – ocram Dec 25 '12 at 18:08
So if I'm doing it for $i = i$, my first equation should read the same, just without the summation sign? – Kaish Dec 25 '12 at 18:12

This isn't a stats question but a question relating to basic properties of calculus and algebra.

It may help to consider a simpler problem, to avoid any confusion about the issue:

$$\frac{\partial{}}{\partial{p_i}} \sum_{i = 1}^n p_i^2$$

Think of the summation written out:

$$\frac{\partial{}}{\partial{p_i}} (p_1^2 + p_2^2 + ... + p_{i-1}^2 + p_i^2 + p_{i+1}^2 + ... + p_n^2)$$

Take the derivative term by term:

$$= \frac{\partial{p_1^2}}{\partial{p_i}} + \frac{\partial{p_2^2}}{\partial{p_i}} + ... + \frac{\partial{p_{i-1}^2}}{\partial{p_i}} + \frac{\partial{p_{i}^2}}{\partial{p_i}} + \frac{\partial{p_{i+1}^2}}{\partial{p_i}} + ... + \frac{\partial{p_{n}^2}}{\partial{p_i}}$$

Now take those derivatives (leaving the $i^{\rm{th}}$ term unevaluated for the moment):

$$= 0 + 0 + ... + 0 + \frac{\partial{p_{i}^2}}{\partial{p_i}} + 0 + ... + 0$$

and we now see why the summation disappears - there's only one term that isn't zero:

$$= \frac{\partial{p_{i}^2}}{\partial{p_i}} = 2p_i$$

Your question is the same but with a different, slightly more complicated function.

Regarding the original problem:

$$l(p_i,y_i) = \sum_{i = 1}^n \left( \ln(p_i) + y_i \ln(1 - p_i) \right)$$

is fine, but as soon as you took derivatives, you went astray.

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