Data Sufficiency questions pose a special challenge to most GMAT test takers.
Read about the common mistakes in Data Sufficiency questions and learn to avoid them.

The following are the common mistakes test takers make in data sufficiency questions. Some other types exist but are quite rare and can be completely eliminated using good practice material. 

1. Forgetting to ignore statement (1) while checking statement (2). This mistake points to answer choice B instead of C. 
   
   Example: 
   At Papertown, all residents buy either newspaper A or newspaper B. How many of the residents of Papertown bought newspaper B? 
   
   (1) Of the 125 Papertown residents, 20% bought newspaper B. 
   (2) 87 people bought newspaper A. 
   
   Here, the number of residents is supplied in statement (1) only, and not in the question itself. When checking statement (2) alone, some test takers apply the total number of residents supplied in the first statement (125) to the second statement to conclude that 38 people (125-87) bought newspaper B.
   However, the total number of residents given in the first statement must not be used for the second statement. The answer should be A and not D.
   
   
   
2.  A statement that proves that the question is incorrect is sufficient to answer the question. Yes or no are legitimate answers. This error might lead to the conclusion that the statement is not sufficient when it is. 
   
   Example: 
   Did more than 25% of the 120,000 students choose college A? 
   
   (1) 63% of the students chose college B. 
   (2) 27,816 students chose college A. 
   
   In this question, the first statement might lead to an assumption that there are only two colleges, which of course might not be true. Therefore, statement (1) is insufficient to answer the question. 
   After a short calculation (not needed here) it is clear from statement (2) that the 27,816 students that chose college A are less than 25% of the 120,000 students. Some test takers make a mistake here by saying that since the A college was not chosen by more than 25% of the votes, statement (2) is insufficient to answer the question. Remember that it does not matter whether the college was chosen by more than 25% of the students or not.  It only matters if the data in statement (2) is sufficient to answer the question. 
   Clearly, statement (2) is sufficient, and the answer is B.
   
   
   
3.  Forgetting to check both statements together. This mistake points to answer choice E instead of C.
   Example:
   Mary paid $180 for football tickets. How many of the tickets cost $30?
   
   (1) Every ticket costs either $45 or $30
   (2) More than 3 of the tickets cost $30
   
   Statement (1) leads to three options:
   a. Zero $30 tickets and four $45 tickets add up to $180.
   b. Three $30 tickets and two $45 tickets add up to $180.
   c. Six $30 tickets and zero $45 tickets add up to $180.
   Since there are three different options, it is impossible to answer the question.
   Statement (1) is insufficient.
   Statement (2) can lead to a large number of options since only the price of one type of ticket is given. Statement (2) is also insufficient.
   Here, some test takers quit and choose E as the answer.
   Of course, they did not check both statements together.
   When using both statements, it is evident that the third option from Statement (1) (Six $30 tickets and zero $45 tickets add up to $180.) supports both statements, and the answer is C.
   Always try to find all possible options before deciding. When each statement alone is insufficient, always remember to check both statements together before choosing E.
   
   
   
4.  Checking both statements together before checking each statement alone. This error might point to answer choice C, when it could be A or B or D.
   
   Example:
   What is the price of 12 nuts and 12 bolts?
   
   (1) 7 nuts and 30 bolts cost $11 together.
   (2) 5 nuts and 5 bolts cost $7
   
   In this question, Statement (1) alone is insufficient to solve since it has two unknowns and no correlating ratio with what is asked.
   Some test takers decide here that since Statement (2) also has two unknowns, it also has no correlating ratio with what is asked. Consequently, they jump to using both statements together, and choose C as their answer.
   Of course, a closer look at Statement (2) shows the same ratio as in the question itself:
   5N+5B=$7. Just multiply it by 2.4 to get 12A+12P=$16.8
   The second statement is sufficient alone, and the correct answer is B.
   
   
   
5. Failing to take into account all of the possible different options that stem from the data.
   
   Example:
   Box A and box B are in a high stack in a warehouse. What is the total number of boxes in the stack?
   
   (1) There are 7 boxes above box A, and 8 boxes below box B.
   (2) There are 3 boxes between box A and box B.
   
   In this question, clearly each statement alone is insufficient to answer the question. Using both statements together, however, seems to some test takers to be sufficient, since it gives data as to how many boxes are above, between, and below box A and box B. They then choose C as the answer. However, there is no data as to which box is below which. Is box A below box B, or the opposite? Since there are two different options for the total number of boxes in the stack, the answer is E.
   
   
   
6. Failing to solve equations or inequalities all the way might lead to various mistakes. There are many common mistakes in equations and inequalities that can be prevented by solving all the way even though this is not mandatory in data sufficiency questions: 
   
   a. Some equations could lead to answers such as, “Every X or no X can solve the equation" - Since two equations with two unknowns can actually be the same equation in disguise.
   b. A quadratic equation might have two, one, or no solutions. In some cases, it is not possible to determine the number of solutions beforehand.
   
   In order to avoid these common mistakes, the question should be solved just as a problem-solving question would be solved. Solve to make sure an answer can be reached.
   
   Example:
   What is the value of X?
   
   (1)  (X-2)/(Y+3) = 4/5
   (2)  4/(X-2) = 5/(Y+3)
   
   Statement (1) alone is insufficient since it has two unknowns. Statement (2) alone is also insufficient for the same reason. Using both statements, we get two equations with two unknowns. Some test takers might choose C as the answer without solving. However, the two equations given are the same equation in a different arrangement.
   The correct answer here is E. 
    

Remember, usually there is no need to solve Data Sufficiency questions all the way. All you have to check is whether there is enough data to solve.
Solve all the way only to make sure you made no mistake.