Average of a posts meta

Hi,

How can I get the average of a posts meta?

I searching around the net – but with no useful result. Maybe the help is here?! Please give me some links or a answer of my problem, if you have.

Here is my query:

$average_args = array(
          'post_type' => 'post' ,
          'post_status' => 'publish' ,
          'cat' => 129 ,
          'meta_key' => rating' ,
          'meta_value' => ''
);

$get_average = new WP_Query( $average_args );
//$average = $get_average->post_count;

Thanks ??