these are unwanted variables, that could affect the DV (other than the IV). They need to be controlled. They can either be participant variables or environmental variables
Participant variables
this is where variables to do with the participants affect the DV, such as:
age
gender
fitness levels
IQ
eyesight
dyslexia
Environmental variables
this is where variables to do with the environment can affect the DV, such as:
distractions
temperature
time of day
noise levels
Confounding variables
these are extraneous variables which have not been controlled, so they do affect the DV. This makes it difficult to say the IV is the only thing affecting the DV
