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How much money is a simple UI change worth?

If anyone doubts that user interface choice can have an impact in the millions of dollars, consider this empirical study on New York City cabs (inspiration: Joshua Gross, May 2012):

The average New York City taxi cab driver received $90,747 per year from fares (gross revenue) in 2004. There are more than 13,000 cabs in the city. In 2007, NYC forced cab drivers to begin taking credit cards, which involved installing a touch screen system for payment.

NYC taxi default tips screen

NYTimes reported in 2009 that "tips, which hovered around 10% when cab rides were cash only, averaged 22% on credit-card transactions this fall".

Why did this happen?

The increase in tips, however, may have less to do with New Yorkers' generosity than with the preset amounts suggested to passengers on the taxi's software systems. In many of the city's cabs, riders are offered options for their tip depending on the length of the ride. For fares under $15, a screen prompts tips of $2, $3 or $4; the numbers can range from 15 percent to 30 percent for higher fares. The presets are used about 70 percent of the time, according to industry estimates.

In September 2009, 28% of cab fares were being paid by credit card.

How much was the credit card change all worth?
Cab driver gross revenue from fares, per year $90,474
Drivers that drive a cab per day (cabs run 24/7) 3
Cabs in NYC 13,000
Difference in tip percentage 12% (22%-10%)
Percentage rides paid by credit card 28%
Total difference in tips thanks to three buttons $118,557,129

Those three default buttons resulted in nearly $120 million of additional tips alone. Per year. Those are some very valuable buttons.

Kickstarter's smart use of UI on the rewards screen

Take a look at the screenshot below. It's the page where you contribute to a Kickstarter project, the most popular crowdfunding platform, with about 3 million users as of June 2012.

Kickstarter's contribution screen

And here's the IndieGoGo perk screen. Yes, it's long. Scroll down. You have to, if you want to contribute.

IndieGoGo's contribution screen

Which design do you think will get more users to contribute more than the perk's value?

Which emphasizes how much you want to contribute, not how much you want to pay to get something?

As far as I know, Kickstarter hasn't published statistics on how many users pledge more than the value of the reward they select. Among Kickstarter's blog posts in the "data" category, I've seen only one reference to this, but it was for the particular case of a $1 reward to buy a CD.