Three Basic Principles of Quantum Mechanics
  1. The event probability P in an ideal experiment is given by the square of the absolute value of complex number which is called the probability amplitude (or shortly "Amplitude") :

  2.  
                       P  =  ½f½2  =  φ · φ*
     
  3. If there are few possible events then can be applied so called superposition principle (linear combination):

  4.  
                             f  =  φ1 + φ2
                       P = ½f½2 = ½f1 + φ2½2 
            ←  Interference
     
  5. If one carry out an experiment in which it is possible to choose alternative decision then the total probability is as follows:
     
                      P = P1 + P2  = ½f1½2 + ½f2½2    ←  no Interference


 
Contrary to classical description of nature we can only talk about the event probabilities in quantum mechanics.