Data Sufficiency (DS) questions in Data Interpretation (DI) sections of competitive exams like the CAT test your ability to determine if the given information is sufficient to answer a question, rather than actually solving it. This skill is crucial for efficient time management and accurate problem-solving.

In a typical DS question, you are presented with a question followed by two statements, labeled (1) and (2). Your task is to decide whether the statements provide enough information to arrive at a unique answer. The options are usually:

A. Statement (1) alone is sufficient, but statement (2) alone is not sufficient. B. Statement (2) alone is sufficient, but statement (1) alone is not sufficient. C. Both statements (1) and (2) together are sufficient, but neither statement alone is sufficient. D. Each statement alone is sufficient. E. Statements (1) and (2) together are not sufficient.

The key to excelling in DS questions is to avoid solving the problem completely unless absolutely necessary. Instead, focus on what each statement allows you to deduce.

Understand precisely what the question is asking. Identify the variables, the relationships, and the target value or condition. Sometimes, the question itself might reveal that certain information is inherently insufficient.

Assume statement (1) is true. Can you answer the question with certainty? If yes, and statement (2) alone cannot, then the answer is A. If statement (1) alone is insufficient, proceed to evaluate statement (2).

Assume statement (2) is true. Can you answer the question with certainty? If yes, and statement (1) alone was insufficient, the answer is B. If both (1) and (2) alone were insufficient, proceed to the next step.

If neither statement alone was sufficient, combine the information from both statements. Can you now answer the question with certainty? If yes, the answer is C. If even with both statements combined, you cannot arrive at a unique answer, the answer is E.

This case (Option D) is often overlooked. It applies when both statement (1) alone and statement (2) alone are sufficient to answer the question. You would typically identify this early if you find that statement (1) is sufficient, and then later find that statement (2) is also sufficient independently.

One common mistake is to start solving the problem with the given data. Remember, the goal is sufficiency, not the final answer. Always test for sufficiency first.

Another pitfall is assuming that if a statement provides a number, it's automatically sufficient. The number must be relevant and lead to a unique answer for the specific question asked. For example, knowing the total number of students doesn't help if the question asks for the average score of boys.

When dealing with DI, the statements will often refer to specific parts of a chart (bar graph, pie chart, line graph) or a table. You need to assess if the information provided in the statement, when combined with the chart/table, is enough to answer the question. For instance, a statement might give the total sales for a year, while the chart breaks down sales by quarter. If the question asks for total sales, the statement alone might be sufficient. If it asks for sales in a specific quarter, you'd need to see if the statement combined with the chart provides that specific detail.

Consistent practice with a variety of DS questions across different DI formats (tables, bar graphs, line graphs, pie charts) is essential. Pay attention to the wording of both the question and the statements to avoid misinterpretations.