The Clinton campaign is not alone in questioning whether an agreement can be enforced, given its voluntary nature.

Due to the voluntary nature of the group its operation was sometimes sporadic.

This argument misses on the voluntary nature by which the industry is created.

Key to the agreement is the voluntary nature of the competition.

Family support programs value the voluntary nature of articipation in their services.

His career shows, yet again, the voluntary nature of religious orthodoxy in the modern world.

Many health professionals are not happy with the voluntary nature of the deal.

"There is no question this report will push the voluntary cooperative nature," he said.

The voluntary nature proposed by the Commission has been endorsed.

I also think it is right that we will be retaining the voluntary nature of the scheme in future.