DNA markers between a suspect and a crime scene specimen are compared, including <span>DNA
minisatellites, short tandem repeats (STRs) and single nucleotide polymorphisms
(SNPs). The total markers that can be compared can number in the thousands, and therefore the chances of DNA fingerprinting identifying the incorrect suspect are negligible. </span>
Uhm... I believe you mean a situation when we examine a relation between two variables. In this case all other conditions must remain as constant as possible because they could affect a result and then the conclusions could be false. ie. you examine a relation between temperature and frequency of cell division/organism's activity, then you should stabilize air pressure, food composition, bacterial flora in experimental room... etc. of course when you conduct research in a field, ie. ecological research you can't do any of that but then you are obliged to explain possible aberrations of your results in an article's discussion.