title: How would I be able to track change based on time? Identify when the condition first appeared - R
body: <p>If I have a data set with different cases over time, what would be the best way to track change?</p>
  <p>Let's say I tracked if there color change in walls over time; think weather damage. I was never consistent in the day or time I started tracking or tracked it. I have duplicate rows in terms of walls because each row represents the measurement at a specific time.</p>
  <p>What would be the best way for me to identify which walls' changed at some point?</p>
  <pre><code>Wall   Color   Date
  A      yellow  2019-08
  A      white  2019-02
  A      yellow    2019-05
  A      yellow  2019-05   
  A      white  2019-04
  A      white  2019-03
  A      yellow  2019-08   
  B      yellow  2019-09
  B      white  2019-05
  B      yellow  2020-09
  B      yellow  2020-05
  c      white  2019-05
  c      white  2018-01
  c      white  2020-06
  c      white  2019-02
  c      white  2020-03
  c      yellow  2020-09
  c      white  2020-06
  c      yellow  2020-05
  c      white  2019-01
  c      yellow  2020-1
  </code></pre>