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The factors are that the person who committed the act was an employee and that they committed the act while they were in the course of business and in the scope of their employment acting on behalf of the employee.
The main defenses to this is that there was no employer/employee relationship, basically that the individual who committed the tort is not an actual employee and does not work for the company.
The second defense would be that the employee was not acting in the scope of the business when the act was committed. For example, if an employee is using a company car to go to a job site but rather goes on a joy trip to the mall instead, and he gets into an accident, that is not acting in the scope of the business and would be a good defense.