Glossary
External validity.
Direct answer
External validity describes how far a study result transfers to other animals, other yards and everyday conditions. It depends on how closely the population, environment, care and measurement methods of the study resemble the target situation. A study that is internally faultless may still carry low external validity.
Internal and external validity often pull against each other: the more tightly a study controls its conditions, the cleaner the causal claim and the less the setting resembles daily life. Before transferring any finding, it is worth asking exactly who was studied, under which conditions and over what period of time.
In horses external validity shifts quickly, because breed, use, feeding, ground type and shoeing vary widely: a result from trained racehorses says little about older leisure horses kept at grass. Transfer from laboratory species or from human medicine counts as a hypothesis to be tested, never as proof in itself.
The evidence letter
Once a month: new syntheses, checked studies, and what has changed in the state of the evidence.