title: Probability over time period
body: <p>Can someone help me with this probability question.  Lets say I know the following:</p>
  
  <ul>
  <li>Probability of event from time period 1 to period 2 (x)</li>
  <li>Probability of event from time period 1 to period 3 (y)</li>
  </ul>
  
  <p>What would be the probability of time period 2 to period 3?  </p>
  
  <p>Thanks in advance?
  John</p>