    [5.5, 15.2]</span></span>
<span><span class="co">## </span></span>
<span><span class="co">##  Ho: difference in  means = 0 ; </span></span>
<span><span class="co">##  Ha: difference in  means != 0 </span></span>
<span><span class="co">##  t = 4.194 , df = 721 </span></span>
<span><span class="co">##  Pr(|T| &gt; t) =  3.08428e-05</span></span></code></pre>
<p>In addition to using <code>by</code>, we can also run two-sample
tests by simply providing two data vectors:
<code>ttest(mri$ldl[mri$sex == "Femal