An arbitrator reviewing the fairness of a dismissal must weigh all relevant aggravating and mitigating factors together in light of the seriousness of the breach. Failure to consider important mitigating circumstances constitutes a failure to understand the nature of the dispute and to deal with its substantial merits, rendering the award reviewable. Even where trust has been affected by an employee's misconduct, dismissal does not automatically follow - all circumstances of the case must be considered. An arbitrator who glosses over and fails to determine important facets of a dispute may be found to have not applied their mind, constituting a reviewable irregularity. The test for review is whether the decision is one that a reasonable decision-maker could have reached based on the totality of the evidence and circumstances.