title: xgboost : The meaning of the base_score parameter
body: <p>In the documentation of xgboost I read:</p>
  
  <blockquote>
    <p><strong>base_score</strong> [default=0.5] : the initial prediction score of all
    instances, global bias</p>
  </blockquote>
  
  <p>What is the meaning of this phrase?  Is the base score the prior probability of the Event of Interest in the Dataset?  I.e. in a dataset of 1,000 observations with 300 Positives and 700 Negatives the base score would be 0.3?</p>
  
  <p>If not, what it would be?</p>
  
  <p>Your advice will be appreciated.</p>