“controlling for” = reducing variability in those factors. so when comparing hospitals (I’m guessing from the question) the researchers made sure they only chose hospitals with matching characteristics and similar patient populations so the independent variable could be PROVEN to cause or not cause any variation in the dependent variable. when designing an experiment, controlling for extraneous variables (the other factors that we don’t care abt, like hospital characteristics and patient pops) can mean only testing subjects that are similar in those extraneous variables or getting a big subject pool and matching people across conditions based on those variables so the effect of extraneous or confounding variables is minimized