Broadly, the pitfalls of scientific procedure are fourfold, as is shown when an endeavor is made to answer the following questions: 1. Are all the necessary phenomena accurately observed? 2. Are all the phenomena relevant to the problem under consideration? 3. Are all the phenomena irrelevant to the problem eliminated? 4. Has the actual sequence or interrelation of phenomena been established?
These difficulties are common to both observation and experiment, but each of these methods also has defects and advantages which are peculiar to itself. According to John Stuart Mill1:
The first and most obvious distinction between Observation and Experiment is, that the latter is an immense extension of the former. It not only enables us to produce a much greater number of variations in the circumstances than nature spontaneously offers, but also, in thousands of cases, to produce the precise sort of variation which we are in want of
LEVINE S. CONTRACTIONS OF GALLBLADDER SEEN IN MAN. Arch Intern Med (Chic). 1927;40(4):420-433. doi:10.1001/archinte.1927.00130100024002