The rationale sentence is where you, as the writer, provide elaboration or interpretation of the evidence.  How does your evidence prove your claim? In your rationale:

• You will explain the significance of  your evidence--why is your evidence important? 
  
• You will make connections, offer insight and demonstrate your critical thinking about your claim and your evidence. 
• You should not simply restate your claim or your evidence. 
  
• You often will consider other information from your source in your rationale, not as a quote, but in elaborating on your evidence.

Because this sentence is so important, you most often will use a compound or complex sentence structure.