Offer your thoughts or opinion on the input financial query or topic using your financial background.

Formula that predicts whether one is better off investing or paying down debt
Although I don't think you need to factor in risk tolerance to get the probabilities, I agree with JoeTaxpayer that you will need to factor in risk tolerance in order to make a practical decision about what to do.  In fact, I think that to make a practical decision you will need more than the specific probability you ask for you in the question; rather, you would like to see the complete probability distribution of possible outcomes. In other words, it's not enough to know that there is a 51% chance that investing will outperform paying down debt.  You actually need to know much it outperforms when it does outperform, and how much it underperforms when it underperforms.  As JoeTaxpayer's comment suggests, you might not choose to make an investment that had a 99% chance of outperforming debt payment by 1%, and a 1% chance of underperforming by 99%. I think it possible to address these questions by doing simulations.  This can be done even with a spreadsheet, but more flexibly with simple programming.  Essentially you can create some kind of probabilistic model of the various factors (e.g., chance that your investment will go up or down) and see what actually happens: how often you lose a lot of money, lose a little money, gain a little money, or gain a lot of money.  Then based on that you can consult your inner spirit animal to decide whether the probability distribution of possible gains outweighs that of possible losses.