title: Group rows by attributes
body: <p>I have a data frame containing data about student lateness to various classes. Each row contains data about a late student and his class: date and time of the class, name of the class, class size, number of minutes late, and the gender of the student. In order to get the total percentage of late students for all classes, I need to count the number of rows (late students) and compare that with the total number of students that attended class.</p>
  
  <p>I can't simply sum the class sizes for all of the rows; that would count the students of a given class several times, once for each late student in the class. Instead, I need to count each class size only once for each meeting of the class. </p>
  
  <h3>Example</h3>
  
  <p>Key: minutes late, class name, students in attendance, gender of tardy student, minutes late.</p>
  
  <pre><code>11/12/10 Stats 30 M 1
  11/12/10 Stats 30 M 1
  11/12/10 Stats 30 M 1
  11/15/10 Stats 40 F 3
  11/15/10 Stats 40 F 3
  11/15/10 Stats 40 F 3
  11/16/10 Radar 22 M 2
  11/16/10 Radar 22 M 2
  11/16/10 Radar 22 M 2
  11/16/10 Radar 22 M 2
  11/16/10 Radar 22 M 2
  </code></pre>
  
  <p>In this case, there are three different class meetings and 11 late students. How could I make sure each class meeting's class size is only counted once?</p>