title: Creating cohort-style data frame from set of observations
body: <p>I'm new to R and have a simple question, as I'm still learning the style of R data manipulation/management. </p>
  
  <p>I have a dataset of observations of basic clinical features (blood pressure, cholesterol, etc) over a period of time. Each observation has a patient ID and date, but are entered as separate line items. Something like this: </p>
  
  <pre><code>Patient ID    Date  Blood Pressure
           1 21/1/14             120
           1 19/3/14             134
           1  3/5/14             127
  </code></pre>
  
  <p>I want to transform the data such that for a given variable (e.g. blood pressure), I have a data frame with one line per patient and all of the blood pressure values observed throughout time in chronological order. Something like this:</p>
  
  <pre><code>Patient ID BP1 BP2 BP3 
           1 120 134 127
  </code></pre>
  
  <p>I want to do this because I want to be able to write code to select the mean of the first three observed blood pressures, for example. </p>
  
  <p>Any advice or reading recommendations would greatly be appreciated.</p>