<p>a function that specifies how to handle observations
containing <code>NA</code>s.  The default action (<code>na.omit</code>,
inherited from the 'factory fresh' value of
<code>getOption("na.action")</code>) strips any observations with any
missing values in any variables. Using <code>na.action = na.exclude</code>
will similarly drop observations with missing values while fitting the model,
but will fill in <code>NA</code> values for the predicted and residual
values for cases that were excluded during the