The district court abused its discretion in excluding the testimony of plaintiff’s fire investigator expert witness. Defendant conceded that the expert was qualified and used generally accepted methodology. The district court improperly found there was too big a gap between the facts on which the expert based his opinion and the conclusions he drew from those facts. The district court improperly weighed the expert’s opinion against testimony of other witnesses to the fire and other experts rather than addressing the factual support the expert cited for his opinion. Since plaintiff based his case on the expert’s opinion, exclusion of the opinion was prejudicial, leading the district court to improperly enter summary judgment in defendant’s favor.