> In the specific example provided. Yes, of course it is. > Rainwater is close to pure water, drinking distilled water distroys cells in gut and causes diarrho

> In the specific example provided. Yes, of course it is. > Rainwater is close to pure water, drinking distilled water distroys cells in gut and causes diarrhoea, same as cholera does. Therefore this is a basic source of error in the experiment that should be controlled. 1. Where did you get this bizarre claim? They sell distilled water in stores for drinking. I've drunk it. It didn't cause me diarrhea, certainly not "same as cholera does". Cholera causes such severe diarrhea that people die of dehydration from it. There's some evidence that it slightly leaches minerals from your body, but that's a far cry from it causing such severe diarrhea that it might literally kill you. 2. Even if what you are saying were true (which is isn't), that would make it a confounding factor or regressor, not a source of error. 3. And even if it were a regressor, that doesn't invalidate the experiment. You can still draw a correlation between hydration and cholera, you just can't conclude that hydration causes cholera. Which is fine, because almost any experiment you do is going to require further experimentation to be able to establish causality. Put another way, this experiment asks the wrong question, but asking a bunch of wrong questions is part of the process of science. The cause of cholera has been known for about a century, so it's easy in hindsight to design an experiment that proves cholera is caused by microbes in fecally-contaminated water. But that's not how science works. In science you're asking questions where you don't know the answer, so you start by asking very narrow questions, like "Is cholera correlated with hydration?" and slowly expanding to questions like "Is cholera still correlated with hydration if you restrict hydration as an intervention?" or "Is cholera still correlated with hydration if you control for water source?" All of these are valid questions around which you can design valid experiments. You just have to understand that when you ask a narrow question you get a narrow answer. None of these experiments conclusively identify the cause of cholera--even the experiment you proposed.