I cannot understand causation and I would really appreciate an experts' help.
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To my understanding causation is a statement like this:

Smocking causes cancer.

and can be diagrammed like this:

Cause------------>Effect
Smocking -------->Cancer

One of the ways to attack a causation that is found in an argument, is to find an alternative cause for the same effect. For example, a statement like:

"Drinking Coca-Cola causes cancer", will weaken our conclusion.

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Now lets examine the following:

The number of people who use X cruise line to go to the islands, has decreased by 5 %. Therefore, this decrease must have had a negative impact on cruise X earnings.

We know as a fact that the number of people has decreased and we speculate that it will have a negative impact on cruise X earnings.

if I assume that this argument has a causal conclusion, it would be like this:

Cause-----------------------------> Effect
decrease in number of people ---> negative impact on X

My question is if an AC that will provide another reason for the negative impact will weaken the argument.

For example, "Line cruise X has cut many cruises, which were usually profitable"

Depending on the answer I have a following question.

I would really appreciate a prompt response, if possible.

Thanks

I look at the argument you have presented about the cruise line more as a logic gap type of argument. Let's examine:
Premise: The number of people who are cruising with company X has decreased by 5%.
Conclusion: This decrease must have had a negative effect on company X's earnings.
Assumption: There is a direct relationship between the number of people cruising on X and the earnings. (I am connecting the two points in the premise and conclusion)

If I were going to weaken this argument, I would attempt to persuade that the opposite is true: that the number of people cruising on X does NOT necessarily affect earnings. For example, here are some possible weaken answers:

A) The average passenger cruising with company X is spending 50% more in on-board purchases than those passengers cruising in previous years. (passengers left are more than picking up the slack in producing revenue)
B) This year cruise line X decided to ban children from its cruises because they did not pay full fare nor did they purchase alcohol or gamble while onboard. (implies that the 5% may have been children who were unprofitable passengers)

I hope this helps! :-)

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Jamie Nelson
ManhattanGMAT Instructor