Production manager Mike Deville is investigating whether a new worker training program improves build $($assembly$)$ times at his factory. The training program involves a $2$ week long intensive session that would cost Mike thousands of dollars. In order to investigate whether the training program is beneficial, Mike randomly picks $15$ of his workers to undergo the training program. He measures each worker's productivity $($units assembled per day$)$ before they undergo the training. After the workers have undergone their training, he re$-$measures each worker's productivity. What statistical test must Mike use to test his hypothesis.