Compare these two statements:
1) There is no evidence of a connection between MMR and Autism.
2) There is evidence of no connection between MMR and Autism.
Do they mean the same thing or something different?
In fact what exactly do these statement mean? This may seem like a trivial question, I have certainly spent most of my adult life thinking the answer was obvious. But it has recently dawned on me that this is not so. There are in fact two subtlety different definitions of “evidence”, and which one you adopt has a profound effect on the meaning of the statements.
In Cambridge evidence is defined as “one or more reasons for believing that something is or is not true”. It is data on which to establish truth or falsehood. This is certainly how I think of scientific evidence – it’s the collection of observations, experiments, and data which you use to decide if a proposition is true or false. With this definition statement (1) means “no data exists, which could be used to determine if there is a connection between MMR and Autism”. Statement (2) means “data exists which shows there is not a connection between MMR and Autism.”
In Oxford they have a different definition of evidence “the available body of facts or information indicating whether a belief or proposition is true or valid: the study finds little evidence of overt discrimination.” In Oxford evidence is only evidence when it purports to the truth of a statement – data about the falsehood of a statement is not evidence at all. With this definition both statements (1) and (2) mean “data has been collected, but it shows there is no link between MMR and Autism.” This is certainly a common usage of the word evidence, see for example , , , .
When I think about it, I find the Oxford definition of evidence utterly bizarre. Why should it matter if the data confirms or denies a proposition? Surely it’s all just evidence. It is in fact so stupid I cannot conceive that anyone really believes this definition, it’s just a slightly sloppy dictionary definition.
The problem is people, myself included, keep saying “there is no evidence that….” It just sounds right. It is unfortunate that it is so wrong – in almost all cases there is in fact heaps of evidence.