'>obs</span> <span class='kw'>=</span> <span class='fu'>c</span>(<span class='fl'>1.2</span>, <span class='fl'>4.8</span>, <span class='fl'>2.5</span>, <span class='fl'>NA</span>))
<span class='no'>df</span> <span class='kw'>%&gt;%</span> <span class='fu'><a href='http://dplyr.tidyverse.org/reference/group_by.html'>group_by</a></span>(<span class='no'>id</span>) <span class='kw'>%&gt;%</span> <span class='fu'><a href='http://dplyr.tidyverse.org/reference/mutate.html'>mutate</a></span>(<span class='kw'>obs_i