de>Mean</code> under <code>Summary</code>) and a 95% confidence
interval for the true mean LDL.</p>
<p>If instead we wanted a two-sample t-test of whether the difference in
mean LDL between males and females were zero, we could stratify using
the <code>by</code> argument:</p>
<div class="sourceCode" id="cb6"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="fu"><a href="../reference/ttest.html">ttest</a></span><span class="op">(</span><span class="va">mri</span><span class="op