These three main categories of epidemiological study may each require, as a preliminary phase, a fourth and rather different approach-that is, a method study.
Comment: in a previous epidemiological study, the prevalence of dysmenorrhea was inversely associated with dietary intake of both fish oil and vitamin B12.
Whether or not the cleaning products are a direct cause of asthma, or simply a trigger for people who already have the disease, isn't clear from this epidemiological study.
