Objective of weaken and strengthen questions

The argument provided in the passage contains a conclusion. Here's what we will be asked to do:

Weaken question: What, if true, would make us question the conclusion given

Strengthen question: What, if true, would make the conclusion given more believable

Irrespective of whether we are dealing with a Weaken the argument or Strengthen the argument question, our basic thought process is the same.

(1) Clearly understand the conclusion that we are trying to weaken or strengthen. Re-read it if necessary.

(2) Look for the reasoning behind the conclusion. What led the author of the argument to arrive at that conclusion? In other words, what forms the basis for the given conclusion.

(3a) For a strengthen question, ask the question: How can I reinforce the basis of this conclusion.

(3b) For a weaken question, ask the question: How can I attack the basis of this conclusion.

Let us undertand this approach better by looking at 3 examples of GMAT Critical Reasoning questions: Easy, Medium, and Hard.

Example 1: Easy GMAT CR Weaken the argument question

Here's a GMAT CR question officially classified as Easy:

"Cocoa grown organically on trees within the shade of the rain forest canopy commands a premium price. However, acquiring and maintaining the certification that allows the crop to be sold as organically grown is very time-consuming and laborious. Meanwhile, the price premium for the grower is about 30 percent, whereas cocoa trees grown in full sun using standard techniques can have twice the yield of organic, shade-grown trees. Financially, therefore, standard techniques are the better choice for the farmer"

Step 1: Conclusion - Standard cocoa is better for the farmer financially versus organic cocoa

Step 2: Basis - Standard cocoa has twice the yield. Also, organic cocoa requires addition expense in terms of cerfitication etc., but at the same time commands a 30% premium over standard cocoa.

Step 3a: Strengthen - Show that standard cocoa (with twice the yield) fetches more income versus organic at 30% premium.

Step 3b: Weaken - Show that organic is still better financially. This can be done either by showing that not all of it can be easily sold so there is wastage etc., or by showing that there are some additional costs involved in growing standard cocoa that reduces margins significantly.

On GMAT, this was asked as a weaken the argument question. The question asked and the options given are:

Which of the following, if true, most seriously weakens the argument?

(a) Cocoa can be grown only in a climate that has the temperature and moisture characteristics of a tropical rain forest.

(b) Cocoa trees grown using standard techniques require costly applications of fertilizer and pesticides, unlike shade-grown trees

(c) Although organically grown cocoa has long commanded a price premium over cocoa grown using standard techniques, its price has fluctuated considerably during that period.

(d) Cocoa is not the only cash crop that can be raised on plots that leave the rain forest canopy overhead esentially intact.

(e) Governments and international conservation organizations are working to streamline organic certification so as to relieve farmers of unnecessary work.

Clearly, option (b) does the job for us. It tells us that standard cocoa requires costly fertilizers and pesticides, which organic do not. This suggests that even with twice the yield, standard cocoa might not be financially better, because even costs are very high.

Example 2: Medium difficulty GMAT CR Strengthen the argument question

Here's a GMAT CR question officially classified as Medium difficulty:

"Budget constraints have made police officials consider reassigning a considerable number of officers from traffic enforcement to work on higher-priority serious crimes. Reducing traffic enforcement for this reason would be counterproductive, however, in light of the tendency of criminals to use cars when engaged in the commission of serious crimes. An officer stopping a car for a traffic violation can make a search that turns up evidence of serious crime"

Step 1: Conclusion - Reassigning police officials from traffic enforcement to higher-priority crimes would be counterproductive

Step 2: Basis - During traffic enforcement checks, stopping & searching a car that was used by the criminal during a serious crime can provide evidence of the crime.

Step 3a: Strengthen - Show that such traffic enforcement action is necessary as it can provide evidence of serious crimes

Step 3b: Weaken - Show that shifting officers from traffic enforcement to serious crimes won't be counterproductive, as traffic enforcement doesn't help much with serious crime prevention or detection.

On GMAT, this was asked as a strengthen the argument question. The question asked and the options given are:

Which of the following, if true, most strengthens the argument above?

(a) An officer who stops a car containing evidence of the commission of a serious crime risks a violent confrontation, even if the vehicle was stopped only for a traffic violation

(b) When the public becomes aware that traffic enforcement has lessened, it typically becomes lax in obeying traffic rules

(c) Those willing to break the law to commit serious crimes are often in committing such serious crimes unwilling to observe what they regard as the lesser constraints of traffic law

(d) The offenders committing serious crimes who would be caught because of traffic violations are not the same group of individuals as those who would be caught if the arresting officers were reassigned from traffic enforcement

(e) The great majority of persons who are stopped by officers for traffic violations are not guilty of any serious crimes 

Option (C) tells us that those committing serious crimes are less willing to obey traffic rules. Hence, the probability of them breaking the rules and getting caught is higher. This can help pick evidence of the serious crime during the search. Hence it clearly strengthens the argument and is the correct answer.

Example 3: Hard difficulty GMAT CR Weaken the argument question

Here's a GMAT CR question officially classified as Hard:

"Typically during thunderstorms most lightning strikes carry a negative electric charge; only a few carry a positive charge. Thunderstorms with unusually high proportions of positive-charge strikes tend to occur in smoky areas near forest fires. The fact that smoke carries positively charged smoke particles into the air above a fire suggests the hypothesis that the extra positive strikes occur because of the presence of such particles in the storm clouds"

Step 1: Conclusion (Hypothesis) - Storm clouds near forest fire sites contain more positive charge because of smoke (presence of such particles) leading to extra positive lightning strikes in such areas 

Step 2: Basis - Most lighting strikes carry negative charge but those occuring in smoky areas are unusually positively charged. Smoke carries positively charged smoke particles into the air above a fire.

Step 3a: Strengthen - Show that the positive charge is indeed on account of the smoke and not some other factor

Step 3b: Weaken - Show that the positive charge may not be on account of smoke and there there is some other possible reason

On GMAT, this was asked as a weaken the argument question. The question asked and the options given are:

Which of the following, if discovered to be true, most seriuosly undermines the hypothesis?

(a) Other kinds of rare lightning also occur with unusually high frequency in the vicinity of forest fires

(b) The positive-charge strikes that occur near forest fires tend to be no more powerful than positive strikes normally are

(c) A positive-charge strike is as likely to start a forest fire as a negative-charge strike

(d) Thunderstorms that occur in drifting clouds of smoke have extra positive-charge strikes weeks after the charge of the smoke particles has dissipated

(e) The total number of lightning strikes during a thunderstorm is usually within the normal range in the vicinity of a forest fire 

Option (D) says that drifting clouds of smoke have extra positive-charge strikes weeks after the charge of the smoke particles has dissipated

If these clouds continue to be positively charged weeks after the smoke-induced positivity has gone, there surely must be some other explanation for the positive charge that these clouds carry. This is what we were looking for in the correct answer: something that delinks smoke as being the causative factor for the positive charge.

Hence this is the correct answer.