this is in scientific investigations
1. knowledge of the researcher it's important to consider the researches captivity and capability in the field which he or she wants to discover. in sense it is, proximity of study in order for him or her to do such approach.
2. tool used. making sure whatever measurement the investigator is using should be valid and reliable. avoiding the possible results to become biased and unstable due to errors which should be avoided.
3. sampling method. obtain the samples which are appropriate and that would cover the entire participants studied- not a half but the entirely of what you want to study. entirely means as much as the possible sample covers.
I hope this helps answer your question:-D