title: Does Recall make sense in 2-class classification task?
body: <p>I know that Recall= TP/ (TP+FN)</p>
  
  <p>Suppose I consider recall values for different contexts of a classifier . In one context , I take classA as my positive class and classB as my negative class . In another context , I take classA as my negative class and classB as my positive class.</p>
  
  <p>Let R1 and R2 values be recall values in these contexts</p>
  
  <pre><code>R1=TP1/(TP1+FN1)
  R2=TP2/(TP2+FN2)
  But TP2=FN1 and FN2=TP1
  So R2=FN1/(TP1+FN1)
  So R1+R2=1
  </code></pre>
  
  <p>What's wrong in this argument?[I feel that something is wrong because I saw  classifiers being evaluated using recall ].</p>
  
  <p>Equivalently , If I am asked to compute recall of the classifier , is it R1 or is it R2 ? How do you decide</p>