Probability sampling methods help reduce sampling bias. In clinical research, simple random sampling methods works well for randomised control trials, and systematic sampling methods are easy to implement in cross-sectional observational studies in the clinic setting.

However, to the best of my knowledge, systematic sampling methods assume every nth person selected will elect to take part in the study. In reality, every person sampled still has to provide informed consent, and thus the informed consent process adds a point at which selection bias can be introduced in much the same way as it is introduced through convenience sampling. That is, even if the process of selecting which patients to approach is random, the requirement for informed consent means that the final sample is a self-selected subgroup of those originally sampled (e.g., people with pain are probably more likely to volunteer for a study on chronic pain prevalence than those without pain).

I have tried to find information on this issue, but can only find standalone information on sampling methods or informed consent, and not how the two may interact in some cases. Apologies if my logic on this issue is faulty, but if it is, where is it faulty?