ceilidh.erickson wrote:You're absolutely right.

Given: firms with PRP contracts in place showed productivity per worker on average 13 percent higher than that of their competitors who used more traditional contracts

Conclusion: PRP contracts increase worker productivity

Assumption: Nothing else besides these PRP contracts accounts for the difference in productivity.

If we want to WEAKEN the argument, we want an answer choice that provides an alternative explanation for higher productivity.

(A) Results similar to those cited for the metalworking industry have been found in other industries where PRP contracts are used.
Other industries are irrelevant - we only care about this case.

(B) Under PRP contracts costs other than labor costs, such as plant, machinery, and energy, make up an increased proportion of the total cost of each unit of output.
There is no reason to believe that cost has any effect on productivity.

(C) Because introducing PRP contracts greatly changes individual workers' relationships to the firm, negotiating the introduction of PRP contracts is complex and time consuming.
Irrelevant. This has no bearing on whether the contract itself affects productivity once it is agreed upon and the employees begin working.

(D) Many firms in the metalworking industry have modernized production equipment in the last five years, and most of these introduced PRP contracts at the same time.
This weakens the argument. If the firms that introduced PRPs also modernized equipment, then the increased productivity might be a result of better equipment, and not worker motivation based on the contract. Correct.

(E) In firms in the metalworking industry where PRP contracts are in place, the average take-home pay is 15 percent higher than it is in those firms where workers have more traditional contracts.
This seems related to the question of PRP contracts --> productivity, but it's a trap! As you'll often find on WEAKEN questions, this is an answer choice that does the opposite - it STRENGTHENS the argument. Now, perhaps the workers are not motivated by fluctuating profits, but that wasn't the argument. The argument was that the CONTRACT motivated higher productivity. That contract would include both the PRP provisions as well as presumably the base pay that worked out to 15% higher than average.