Employees may resort to strike in the event of a conflict where other measures have failed. A lockout by an employer is the counterpart to the strike. The right to strike in the private sector is guaranteed under the NLRA. However, only about half of the states extend this right to employees in the public sector. Where public employees are not permitted to strike, state statutes often impose monetary or similar penalties on those who strike illegally. In states where strikes by public employees are permitted, employees must often meet several conditions prior to the strike. For example, a state may require that a bargaining unit has been properly certified, that methods for impasse resolution have been exhausted, that any existing collective bargaining agreement has expired, and that the union has provided sufficient notice to the school board. The purpose of such conditions is to give the parties an opportunity to avoid a strike, which is usually unpopular with both employers and employees.