This, according to Google, is a common question about expert witnesses. The simple answer is…Yes, of course!
That’s the whole point of an expert witness, to provide opinions on a particular subject matter. The expert examines the evidence provided and, based on that evidence, provides an opinion. When being hired by an attorney, in fact, the attorney will say something like 'we want you to opine on x.'
The attorney may even tell you what opinion he or she wants…or more likely will tell you something like 'we believe x happened, so we need you to look at the evidence and tell us if that’s correct.'
One thing you’re not supposed to do, though, is simply give clients the opinions they want regardless of what the evidence shows. Expert witnesses need to provide unbiased, truthful opinions, opinions that are supported by the evidence. (Here’s a tip; if your opinion would be scoffed at by colleagues in your field, you may have tried to be too helpful to your client!)