Critical Reasoning

I am not big fan of formal approach for solving GMAT weakening / strengthening --- a formal approach takes a different path altogether to what GMAT intends to test through such questions on CR. Whole reason of such questions on GMAT is to test common world understanding of things. Weakeners, strengtheners, evaluate the argument, and paradox questions -- if we try to solve them through a formal approach, we might get some right, while we might get many a wrong too. GMAT is exam for future business leaders, who need to be street smart with their logical thinking that is needed when in the business field. Such questions are the way GMAT tests these skills.

Coming to the key here.

There are definitely a few good key takeaways, but do not build such takeaways on a formal deduction grounds.

My suggestions and takeaways:

1) In real life scenario: Take weakener as two people with opposing views discussing on something. While strengthener as two people resonating with each other are discussing on something. Evaluate an argument can be seen as peoples' boss trying to find a balance between these two people who have opposing views. In all cases, you can take person A as the author of the argument, while person B as yourself. As person B, you just need to wear a different thinking hat for different questions -- but there are only 6 thinking hats (Edward De Bono).

2) Arguments talking about a past event can probably be weakened by a different (alternate) reasoning than given in the argument. For an already occurred event, author has specified one reason for the event in argument. Your aim more often shall be to bring down the argument reasoning by providing an an alternate reasoning from your external knowledge (scope being the answer choices), as person B. This works very well in real life scenarios too. Assume, person A says project ABC was successful because Ann worked so hard and did 70% of the project work. As person B, your reasoning can be - but Ann was on vacation for 50% of the project duration - it was his team who made sure Ann's absence does not hurt the project.

3) If some argument is predicting a future event - it would definitely be wrong idea to weaken it with another prediction. It's like Person A says stocks are going to rise for XYZ reason, and then person B says stocks are going to fall for ABC reason -- which one to believe -- no one. That's why more often, for a future event arguments, alternate reasoning might be bad idea. You can weaken by targeting the line of reasoning - giving certain evidence that questions the basis of reasoning on which argument is built. In most cases, you are trying to play smart chap, as person B, here - you are saying to person A: oh, you might be right, but see this is what is happening right now (certain evidence) - then may be what you (person A) said is wrong. In real life scenario, if person A says project ABC will be successful as Ann who has made the last project successful is also part of the team. Person B can not weaken such argument by saying Mary (a new joinee) who has made a successful project in her last company will be the key reason for project success -- both might be right, both might be wrong.
Let's discuss it through some official examples.

Example 1:
A study of marital relationships in which one partner's sleeping and waking cycles differ from those of the other partner reveals that such couples share fewer activities with each other and have more violent arguments than do couples in a relationship in which both partners follow the same sleeping and waking patterns. Thus, mismatched sleeping and waking cycles can seriously jeopardize a marriage.
Which of the following, if true, most seriously weakens the argument above?
(A) Married couples in which both spouses follow the same sleeping and waking patterns also occasionally have arguments that can jeopardize the couple's marriage.
(B) The sleeping and waking cycles of individuals tend to vary from season to season.
(C) The individuals who have sleeping and waking cycles that differ significantly from those of their spouses tend to argue little with colleagues at work.
(D) People in unhappy marriages have been found to express hostility by adopting a different sleeping and waking cycle from that of their spouses.
(E) According to a recent study, most people's sleeping and waking cycles can be controlled and modified easily.

Here choice D, a correct choice, is giving a sound evidence (please note it's not a reasoning) that directly questions the basis on which author has built his argument.

Example 2:
Twelve years ago and again five years ago, there were extended periods when the Darfir Republic's currency, the pundra, was weak: its value was unusually low relative to the world's most stable currencies. Both times a weak pundra made Darfir's manufactured products a bargain on world markets, and Darfir's exports were up substantially. Now some politicians are saying that, in order to cause another similarly sized increase in exports, the government should allow the pundra to become weak again.
Which of the following, if true, provides the government with the strongest grounds to doubt that the politicians' recommendation, if followed, will achieve its aim?

(A) Several of the politicians now recommending that the pundra be allowed to become weak made that same recommendation before each of the last two periods of currency weakness.
(B) After several decades of operating well below peak capacity, Darfir's manufacturing sector is now operating at near-peak levels.
The country won't be able to meet the increased demand for goods if its factories are already operating at a peak. Hence no increase in revenue from exports.
(C) The economy of a country experiencing a rise in exports will become healthier only if the country's currency is strong or the rise in exports is significant.
(D) Those countries whose manufactured products compete with Darfir's on the world market all currently have stable currencies.
(E) A sharp improvement in the efficiency of Darfir's manufacturing plants would make Darfir's products a bargain on world markets even without any weakening of the pundra relative to other currencies.

Here choice B, the correct choice, is giving sound evidence why such a path might not be fruitful.

Example 3:
Kernland imposes a high tariff on the export of unprocessed cashew nuts in order to ensure that the nuts are sold to domestic processing plants. If the tariff were lifted and unprocessed cashews were sold at world market prices, more farmers could profit by growing cashews. However, since all the processing plants are in urban areas, removing the tariff would seriously hamper the government's effort to reduce urban unemployment over the next five years.
Which of the following, if true, most seriously weakens the argument?
A. Some of the by-products of processing cashews are used for manufacturing paints and plastics.
B. Other countries in which cashews are processed subsidize their processing plants.
C. More people in Kernland are engaged in farming cashews than in processing them.
D. Buying unprocessed cashews at lower than world market prices enables cashew processors in Kernland to sell processed nuts at competitive prices.
E. A lack of profitable crops is driving an increasing number of small farmers in Kernland off their land and into the cities.

Here choice E, the correct choice, is giving sound evidence why government decision might not hurt government's plan to reduce unemployment.