> When we randomise people, these influences will still be operating on the outcome, which will vary across the people randomised to our conditions. Does randomisation mean that all these different effects are balanced somehow? No – not least because confounders do not exist in experimental studies! This is for the simple reason that a confounder is something that affects both the exposure and the outcome, and in an experimental (i.e., randomised) study we test for a difference in our outcome between the two randomised groups.
I do not think this sort of word-play is useful. If your random samples are small (and even if statistically adequate) the chance of confounders in one or both groups of an A/B can be relatively high, even though the selection procedure for treatment is random. So, "No – not least because confounders do not exist in experimental studies!" is misleading if it is expected on the basis that randomisation of treatment allocation somehow makes confounding impossible.
That the possibility of confounding is equally likely in both branches remains true for all sample sizes of an A/B where #A=#B and allocation is random. So, in my opinion, not a myth.
I do not think this sort of word-play is useful. If your random samples are small (and even if statistically adequate) the chance of confounders in one or both groups of an A/B can be relatively high, even though the selection procedure for treatment is random. So, "No – not least because confounders do not exist in experimental studies!" is misleading if it is expected on the basis that randomisation of treatment allocation somehow makes confounding impossible.
That the possibility of confounding is equally likely in both branches remains true for all sample sizes of an A/B where #A=#B and allocation is random. So, in my opinion, not a myth.