I work with a fairly large team across departments. One of my coworkers, whom I've never met in person, is polite and responsive. However, the data that they send to me for analysis is rarely accurate. The problems include out-of-date spreadsheets, data for the wrong projects, and most often data that is just wrong (numbers aren't correct for whatever reason).
I generally like this person but their requests take me days instead of hours since I have to correct my results so often - twice on average and on this task 3-4 times. I'm not worried about budgeting since their project budgets are high but re-doing my work constantly is irritating and takes me away from other tasks.
How can I politely ask this coworker to ensure that they're sending me the correct data?
Edit: I should add that I don't have any way of verifying this data. Think of someone reporting rainfall from their back yard - but they don't tell me where their yard is. So the only way that I know something is wrong is when they tell me that they made a mistake and I need to do the analysis again.
Edit2: I requested a double-check of their data and they assured me that they triple-checked it. However, they sent me an email this morning correcting their triple-checked results.