title: Confusion about (Mean) Average Precision
body: <p>In <a href="https://stackoverflow.com/questions/40865645/confusion-about-precision-recall-curve-and-average-precision/40904548#40904548">this</a> question I asked clarifications about the precision-recall curve.</p>
  
  <p>In particular, I asked if we have to consider a fixed number of rankings to draw the curve or we can reasonably choose ourselves. According to the <a href="https://stackoverflow.com/a/40904548/4480180">answer</a>, the second one is correct. </p>
  
  <p>However now I have a big doubt about the Average Precision (AP) value: AP is used to estimate numerically how good is our algorithm given a certain query. Mean Average Precision (MAP) is average precision on multiple queries. </p>
  
  <p>My doubt is: if AP changes according to how many objects we retrieve then we can tune this parameter to our advantage so we show the best AP value possible. For example, supposing that the p-r curve performs wonderfully until 10 elements and then horribly, we could "cheat" computing the (M)AP value considering only the first 10 elements.</p>
  
  <p>I know that this could sound confusing, but I didn't find anything about this anywhere.</p>