In Daubert: Very Convoluted, Usually Confusing to Many, Nevertheless Elegant, Armand Rossetti writes:
Daubert also requires that an expert witness has to assist and not to confuse the Trier of fact. If the Jury (Trier of fact) can understand whether a substance in particular is able to cause an injury and whether an injury resulted from that particular substance, an expert opinion would be redundant and unnecessary.
On the other hand, Rule 403 requires a Court to balance the probative value against the prejudicial effects of expert witness testimony. Courts should not confuse Rule 403 with Rule 703.
For example, experts do not necessarily need an epidemiological study with an internal control group or randomly selected participants to be able to show an association between exposure to a chemical and resulting bodily abnormalities. That is true because an association is not causation. While a case study might not be sufficient to show causation, it may be all that is necessary to show an association between presence of a toxic substance and a resulting injury.