title: How to add a calculated column in a pandas dataframe?
body: <p>I am new to python/pandas so I'm struggling a bit here.
  I have a dataframe with air quality data from 2016 to 2020. I want to calculate the annual rate of change for each measured value to compare them with the value the year before at the same day and month.</p>
  
  <p>These are the first lines of the dataframe.</p>
  
  <pre><code>         Date Country       City Specie count   min   max median variance
  0  2020-02-23      CR  San JosÃ©   pm25    20  13.0  53.0   25.0  1232.00
  1  2020-04-04      CR  San JosÃ©   pm25    23  17.0  57.0   38.0  1302.57
  2  2020-04-24      CR  San JosÃ©   pm25    23  30.0  80.0   59.0  1966.13
  3  2020-01-14      CR  San JosÃ©   pm25    24  13.0  34.0   21.0   379.55
  4  2020-02-07      CR  San JosÃ©   pm25    23  57.0  95.0   72.0   838.97
  </code></pre>
  
  <p>Does anybody have an idea as to how I can proceed?</p>
  
  <p>Thank you </p>