Why does LaTeX tell me that a { is missing?

Question

Whenever I compile my document including the following code, it produces satisfactory output on the pdf, but gives me an error:

43
Missing { inserted.
<to be read again> 
                   \gdef 
l.43 \item $\mu_\hat{p}
                       $ = mean of all possible values of $\hat{p} = \pi$. \\

---

\begin{itemize}
\item $\mu_\hat{p}$ = mean of all possible values of $\hat{p} = \pi$. \\
\item $\sigma_\hat{p}$ = standard deviation of all possible values of \^p =     $\sqrt{\frac{\pi(1-\pi)}{n}}$ \\
\item CLT (for \^p): If $n\pi \geq 10$ \underline{AND} $n(1-\pi) \geq to$, then the histogram of all possible values of \^p will be approximately normal. \\
\end{itemize}

What did I do wrong?

Answer 1

You need braces for the subscripts: \mu_{\hat{p}} and \sigma_{\hat{p}}

\documentclass{article}
\usepackage{amssymb}
\begin{document}

\begin{itemize}
\item $\mu_{\hat{p}}$ is the mean of all possible values of $\hat{p} = \pi$.
\item $\sigma_{\hat{p}}$ represents the  standard deviation of all possible values of $\hat{p} =     \sqrt{\frac{\pi(1-\pi)}{n}}$
\item CLT (for $\hat{p}$): If $n\pi \geq 10$ \underline{AND} $n(1-\pi) \geq to$, then the histogram of all possible values of $\hat{p}$ will be approximately normal.
\end{itemize}

\end{document}

I also changed the text \^p to the math \hat{p} for consistency's sake, and suppressed the \\ at the end of each \item (those produce undefull \hboxes). Perhaps instead of $n(1-\pi) \geq to$ you meant something like $n(1-\pi) \geq t_{0}$?

Underlining it's not a nice typographical feature, so I would also suggest you to suppress it.