Proving that you have a track record of success will take you one step closer to being hired. The STAR method (Situation, Task, Action, Result) works wonders when answering this and a number of other interview questions. Use it to tell the story of your achievement:

☛ Situation: I was a Junior Analyst.

☛ Task: I was asked to take charge of the invoicing process.

☛ Action: Within a month, I streamlined the process.

☛ Result: A reduction of 10% on invoicing errors and 15 man-hours saved that month.