Observations and results
Did the reaction time increase as more distracters were added? Did it take longer for volunteers to answer when the target was absent compared with when it was present?
You should have seen that, in general, the reaction time needed to do the visual search increased as more distracters were added. (There may have been some exceptions, such as a person taking only slightly longer to do a visual search with three distracters present in comparison with four, but it should have clearly taken a good deal more time to find the target when there were four distracters compared with when there was just one.) When more distracters are present, it makes finding the target more difficult. (Think of the example with the red letter T target and letter L distracters that became more distracting when they changed from all blue to half red.) This makes people take more time in their visual search, even if the target is not there. In fact, you should have seen that people actually take more time when the target is absent compared with when it is present, as they may spend more time checking and rechecking to make sure that the target is really not there.
More to explore
Cognitive Science Software: Visual Search, from Tom Busey at Indiana University Bloomington
Research Explains How the Brain Finds Waldo, from ScienceDaily
The Truth Behind Where's Waldo?, from ScienceDaily
The Brains Behind Where's Waldo?, from Science Buddies