Wednesday, March 26, 2014

Explanation of Trip Splitter's Even-Up Algorithm

First of all, a quick overview of how to use Trip Splitter optimally. It is designed to make everything about sharing expenses fast and easy to do. If you are having trouble with anything, you probably aren't using Trip Splitter correctly.

First define a trip and all your participants. (Menu button, My Trips, New Trip)

Then you enter expenses as they occur, indicating who paid and who participated.

At the end of the trip (or anytime to see the current situation), you turn the device sideways and you will see the minimum number of inter-person payments that would even everyone up. These are called "even-up" payments.

You can also send an email at any time which will summarize all of this. The first table in that email details the "even-up" payments. The second table summarizes all the expenses by person and their total amounts of payments and total amounts of usage.

When people first start using Trip Splitter, they sometimes create a small number of expenses and test the even up algorithms.  The even up algorithm sometimes created non-intuitive payments because it is designed to create the minimum number of payments between the parties to even up. The non-intuitive results show up most often when looking at a small number of expenses. You might get a situation where one person is to pay another person and those two people didn't even participate in any of the same expenses.

But it is correct, because Trip Splitter has an algorithm that creates the minimum number of even-up transactions between all the participants, and doesn't just look at the relative usage/payments between specific pairs of people.  Because of user requests we are looking at supporting variations on the even up process, such as taking into account people who are financially coupled like spouses, or making everyone even-up in relation to one other participant, like a trip leader who pays most of the expenses. But right now, no changes are scheduled.

In a recent release, we added an explanation of the even-up process that is displayed in cases of 10 or fewer expense items, to help with the possible non-intuitive results. I'll include that here as an added explanation of what is going on.

If these even up payments do not seem intuitive or accurate, please consider that our algorithm is based on creating the fewest number of inter-person payments. The goal, obviously, is for everyone to pay out exactly the amount of their portion of the expenses they were involved in. Once the totals of everyones' payments and expenses are calculated, the person owing the most is marked to make a payment to the person who overpaid the most, etc. This may result in payments between people who didn't even participate together in a shared expense, and that can be confusing. But you can verify it is accurate by sending an email of the expenses, and the tables there will show how much each person paid and how much each person used of the shared expenses.

As the note mentions, the way to satisfy yourself that everything is accurate is to examine the first two tables in the financial email. The second table shows you how much each person used of all the expenses and how much each person paid toward all the expenses. If those two amounts aren't the same for each person, then they either overpaid or underpaid. The first table shows who would pay who how much to make it all come up even.