[Tex/LaTex] Displaying normal size symbols in formula

equations

How can I make the following part of code: 

\sum_{i=1}^n \sum_{j=1}^n w_{ij}

in normal size and not in subscript size?

\documentclass[a4paper]{article}
\usepackage[english]{babel}

\begin{document}

\begin{equation}
I = \frac{n}{\sum_{i=1}^n \sum_{j=1}^n w_{ij}}
\frac{\displaystyle\sum_{i=1}^n \sum_{j=1}^n w_{ij}(x_i - \bar{x})(x_j -
  \bar{x})}{\displaystyle\sum_{i=1}^n (x_i - \bar{x})^2}
\end{equation}

\end{document}

Best Answer

You should add a \displaystyle directive in the denominator of the first \frac expression:

\documentclass[a4paper]{article}
\usepackage[english]{babel}
\begin{document}

\begin{equation}
I = \frac{n}{\displaystyle\sum_{i=1}^n \sum_{j=1}^n w_{ij}}
    \frac{\displaystyle\sum_{i=1}^n \sum_{j=1}^n w_{ij}(x_i 
      - \bar{x})(x_j - \bar{x})}{\displaystyle\sum_{i=1}^n (x_i - \bar{x})^2}
\end{equation}
\end{document}

Addendum: Technically, without the displaystyle directive, the main elements of expressions in the numerator and denominator of a \frac command that's part of a displayed equation are not in "(sub)script" style but in "text style".

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