testimony of an expert, like that of all witnesses, is to be given only such weight and credit as you think it is properly entitled to receive. now, the jury must determine the credibility of witnesses. in deciding this, you may consider all of the facts and circumstances of the case, the witness imaner of testifying, the witness' means and opportunity of knowing the facts about which they testify, the nature of the facts -- the nature of the facts about which the witness testifies, the probability or improktbability their testimony, the witness' interest or lack of interest in the outcome of the case, the witness' personal kreblgtd insofar as it may have been shown in your presence and by the evidence. any evidence of bias for or against a party, any possible motive in testifying, if shown by the evidence, in that regard