And it's not just rats. Anything with multiple senses, a circadian rhythm, developmental variables, individual genetic differences can be subject to this variab

And it's not just rats. Anything with multiple senses, a circadian rhythm, developmental variables, individual genetic differences can be subject to this variability. These are all factors that people should be taking into account, and yet unfortunately they do not. Some of it is practicality, it's incredibly challenging and expensive to control for everything, so you randomize certain factors instead. Confounding effects can sometimes be small enough to be washed out anyway. Nonetheless, it is known by psychophysicists (people who do their best to measure behavior rigorously) that all of these factors are important. Good behavioral research is incredibly challenging to get right, the best people know this, but most published behavioral research is quite sloppy.