The weaken standard is the most common standard in the evaluating evidence strand. With this task, the LSAT is asking you to identify which answer has a piece of evidence that makes the conclusion less likely to be true. Answer choices will acknowledge the evidence that has been given in the stimulus, but essentially hijack that evidence to lead to a negation of the conclusion. For causal conclusions, this often means that the answer will say that some other cause has led to the phenomenon in the conclusion.
Distractors for weaken questions will often present information that does not address the stimulus’ conclusion; these distractors are often referred to as being "out of scope." Additionally, common distractors will be neutral pieces of evidence that do not make the conclusion more or less likely to be true. Occasionally distractors will be evidence that strengthens the conclusion, and are thus opposite of what the task is asking you to identify.
The question stem for weaken questions are fairly similar, and thus it is easy to identify while taking the LSAT. A common variant is: "Which one of the following, if true, most weakens the argument.” This stem is emphasizing that the evidence in the correct answer is to be taken at face value, no matter how unlikely it may seem given the other evidence in the stimulus.
The real world cognitive task that is tested by the weaken standard is identifying evidence that can either attack an opponent’s argument, or identifying evidence that your argument needs to address in order to be valid. This, of course, is practiced every day by lawyers, we need to evaluate arguments and structure their evidence to make the best case for the client.
Once you've identified that a given question is a weaken task, you should actively read the stimulus and identify the argument’s evidence, bridge, and conclusion. You should be able to draw a mental chart that links the argument's evidence to the conclusion. Once you've identified that link, you can pre-phrase the negation of the conclusion.
For instance: if a stimulus’ conclusion is that reading more books will always lead to an increase in vocabulary, your pre-phrase should be that reading more books will not always lead to an increase in vocabulary. Causal conclusions are typically easier to negate than relative conclusions, but you can easily practice with the following example: Solving poverty is more easily accomplished by the government than the private sector. Although the negation would be "solving poverty is not more easily accomplished by the government than the private sector," that still leaves open the possibility that the routes are equally easily accomplished or that neither route is easily accomplished.
The correct answer choice will acknowledge the evidence from the stimulus, but still lead you to this pre-phrased negation. Thus, you should not try to pre-phrase an actual answer choice. This is because the "if true" evidence in the answer choice could be a multitude of possible pieces of evidence; is not helpful to try and think of all the ways that a given argument could be weakened if some other fact were true.
Once you have this pre-phrase, you can go to the answer choices and actively ask yourself "Combining the evidence in the stimulus and this new evidence, is the pre-phrased negation true?"
Another way of thinking about it is that the new evidence is hijacking the stimulus evidence, and leading it in the opposite direction. For instance: an argument could claim that a country's legislation regulating the safety of workplaces has led to a decrease in employee accidents in manufacturing. The pre-phrase would be that the legislation has not led to a decrease in employee accidents. The correct answer choice could be that increases in automation of manufacturing tasks has led to fewer employees working in manufacturing jobs. This new piece of evidence shows that the legislation is probably not the cause, but rather that there are fewer people at risk of injury due to some external evidence. This external evidence, if true, makes the causal connection between the legislation and the decrease in employee accidents less likely.