In Washington criminal cases where a defendant faces charges stemming from multiple criminal acts extending over a period of time, it's important that jurors be instructed on the specific acts they are considering. Otherwise, there is a risk that juror unanimity could be compromised. That is, some jurors might agree to convict on the basis of evidence presented for one act, while others may convict on the basis of evidence presented for another.
Our Pierce County criminal defense attorneys know this would amount to a constitutional violation. This is why, in the 1984 case of State v. Petrich, the Washington Supreme Court ruled prosecutors may elect which act in a series they rely upon for conviction. Otherwise, a jury should be instructed that all 12 must agree that the same specific underlying act has been proven beyond a reasonable doubt. This is referred to as a "Petrich instruction."
However, there are sometimes exceptions, as illustrated in the recent case of Washington v. Andlovec before the Washington Court of Appeals, Division Three. Jurors in this case heard evidence of many separate sexual assaults, and they were never instructed on which specific act they were considering. Defendant raised this issue on appeal.