Precision = tp/(tp+fp)
Recall = tp/(tp+fn)
tp- Relevant documents retrieved by the system
fp- All documents retrieved by the system, based on the user query
fn – All the relevant documents present
Precision/recall values can be plotted and the area under the Precision/recall curve is a measure of the effectiveness.
Practically, 100% effectiveness is almost impossible to achieve. Why? Because, it is entirely dependent on the User.