That biomedical knowledge has an inborn tendency to decay can be considered a truism. More interesting is: why is decay such an integral part of biomedicine? The reason is almost always due to a displacement phenomenon that inevitably degrades existing knowledge in favor of innovations. Often the lack of decay is the problem, when outdated information ought to be systematically eradicated but instead is preserved long beyond its relevance, ending up as monuments of dead knowledge.
Measuring Decay Rates
Realizing the enormous sum of biomedical knowledge, representing all health disciplines, easily leads to the conclusion that scientific measures of total decay rates are not feasible. Instead decay rates for selected small areas can be measured and might represent approximations of other areas from disciplines other than the one tested.
Dealing with selected gastroenterological areas, I have attempted to measure the decay rates over a period of 38 years, 1946-1984.' Based on the authorized Danish textbook of internal medicine,2 published regularly once or twice each decennium over the period mentioned by the same publishing firm, and based on contributions by leading specialists of Denmark, it was possible to correlate the information of the publication year and that of 1984. The selected topics were gastritis, ulcer disease, gastric cancer, Crohn's disease, and ulcerative colitis. By chopping the corresponding chapters into basic information pieces, it was possible to discriminate between current and obsolete information at the time of evaluation, and even to apply a numerical technique. The results were T 1/2values of 18-36 years, and T 1/4-values of 18-31 years. Expressed as an annual decay rate, it was 0.6-1 percent. The active, postgraduate years of health personnel amount to 30-40 years. In other words, more than half of a young doctor's knowledge would be obsolete by the end of his or her active years if not concurrently restored. Within the topics mentioned, the decay rates are even minimum figures, because some elements of un-