Sometimes we have to make decisions without all of the pertinent information at our fingertips. The interviewer wants to know that you are capable of taking educated guesses and that you are confident enough in your abilities that you can make a good decision without all pieces of the problem being present.
"When I need to decide without all of the information present, I weigh the pros and cons and come up with a solution that makes the most sense. Common sense can take you a long way! Next, I may ask the opinion of someone I trust to see what they think. Even though I trust my decision-making ability, I still think it's important to get a second opinion when it comes to situations involving money or decisions that make a significant impact on others."
"I am a strong believer in following my gut, and for the most part, it has not steered me wrong. I try to gather as much information as possible, but when all of the pieces are not accessible, I assess the situation using my prior knowledge of similar situations, and I follow my intuition. If I'm not certain or feel conflicted, I don't hesitate to bring in another person or persons to help me come to the best, most effective decision."