The best analysis of the difference in the evidence used by each speaker to reach his conclusion is:
Kennedy uses facts to compare U.S. economic growth with other industrialized nations, which show the U.S. is lagging; Nixon uses facts to compare the economic growth during this presidency to that during the previous one, which shows that the economy is improving.
This response accurately captures the distinct approaches taken by Kennedy and Nixon. Kennedy focuses on the U.S.'s performance in a broader international context, highlighting a perceived deficiency compared to other countries. In contrast, Nixon evaluates the current administration's performance against the previous one, suggesting that the economy is on an upward trajectory.