The result of that experiment could mean a variety of things, such as:

  - sample size too small (too few tests,
    too few test subjects)
  
  - poor test setup
  
  - praying for success made her complacent,
    thus not leading to success as often as
    when not praying for success
  
  - praying for oneself is not rewarded but
    punished
  
  - God does not permit scientific experiments
    to prove God's existence, even indirectly
    via measuring the effectiveness of prayer
  
  - other reasons such as your "[o]r God had
    other plans for Cecilia".

A theologist can probably come up with more reasons.

Anyways, that test does not pass the smell test for me.