Sometimes the absence of evidence does amount to evidence of absence. When evidence is methodologically sought for X, and the evidence generated is of the null sort (nothing there), we can place this lack on the scales of rational evaluation. Null news actually does count. Certainly not as strongly as positive findings, but when being rational, this category of finding is important, though largely overlooked.
In a sense, absence of positive evidence is evidence of of absence. A “proving” of a negative. In the case below, the absence is of an effect for a dietary supplement benefit.
A number of psychiatrists and psychotherapists with a holistic streak to their thinking have been recommending that people with depression add fish oil supplements to their diet. In the case of the above study, it has been found that these supplements appear not to work, at least in the case of depression of the postpartum sort.
[A] randomized trial that included more than 2,000 women finds that use of DHA supplements did not result in lower levels of postpartum depression in mothers or improved cognitive and language development in their offspring during early childhood, according to a study in the October 20 issue of JAMA.
Case closed, right? Fish oil is nothing more than a current incarnation of snake oil, yes? Wrong. No. Rather, the above information is simply added to the scales of rationality. It is the totality of information and reasoning we must weigh, not just single stories, studies, or hypotheses.