What is the test for validity testing in medical malpractice cases?

In my opinion, there is nothing objectionable about validity testing per se. It goes to the reliability of the opinion expressed by the expert and the weight to be given to it by the trier of fact. That is a proper purpose. There will be occasions where there is an over-emphasis on the validity testing, and a concern may arise that the jury will use the evidence for the prohibited purpose of oath-helping. On such an occasion, the judge may intervene in the examination of the expert and limit the questioning on the topic. Alternatively, the concern may be addressed by a limiting instruction to the jury by the judge. This latter point was made by Mr. Justice Thackray in Lawson v. McGill, 2004 BCCA 68, 23 B.C.L.R. (4th) 254: