Early research by washburn suggested that stomach contractions caused hunger sensations. later research showed that the presence of the balloon in washburn's stomach had caused digestive contractions, not hunger contractions. this is an example of the effects of variables on scientific research.

These are third (or confounding) variables. These are the variables that are not accounted for in preliminary research that can color the findings and cause them to be invalid or otherwise unrepeatable. These variables can have an effect that would otherwise not be existent had the variables not been a part of the original research.

The answer would be hard press.