Can a party be required to answer interrogatories that require an opinion?

In some circumstances a non-party may be required to answer interrogatories that require an opinion. In some cases the distinction between fact and opinion may not be clear. Gratt v. R (1982), 1982 CanLII 33 (SCC), 31 C.R. (3d) 289 (S.C.C.) Under Rule 19.02 interrogatories are governed by the same general rule relating to scope as applies to oral discovery: see Rule 18.09. The key tests are relevance to the subject matter of the proceeding and that the answer lies within the knowledge or means of knowledge of the witness. When the question calls for an expert opinion it must also be shown that the witness is qualified to answer the question. I will address the additional concern of not unduly burdening the witness in paragraph 21. The question here is whether the opinions sought by the interrogatories in this case are opinions which CPR 19 requires be provided.