‘Supervisory employee’ means any person having authority in the interest of the employer to hire, transfer, suspend, lay off, recall, promote, discharge, assign, reward or discipline other employees or responsibility to direct them, to adjust their grievances or effectively to recommend such action, if in connection with the foregoing, the exercise of such authority is not of a merely routine or clerical nature but requires the use of independent judgment. The exercise of such authority shall not be deemed to place the employee in supervisory employee status unless the exercise of such authority occupies a significant portion of the employee’s workday.