Android is threatening to overtake the iPhone in one important metric

The consensus has always been that the iPhone is the "rich man's phone."

iPhone owners - on average - earn more and spend more on apps and in-app purchases. And accordingly, ad rates are typically higher on iOS. This makes the iOS platform more attractive to app developers, despite Android's larger install base, and helps drive a more vibrant ecosystem.

But times are changing.

Research from mobile tech company Moovweb has found that when it comes to e-commerce on iOS and Android, iPhone users are still slightly more valuable than Android users - but the gap is closing fast.

Android is catching up on conversion rates - and amount spent per session

For example: In Q1 of 2014 there was a 20% difference in "conversation rates" on mobile commerce sites between Android and iOS.

• Gap in conversion rates favouring iOS over Android:
• Q1 2014: 20%
• Q1 2015 5%

A conversion rate is how likely a visitor to a site is "converted" into a sale - if 1 in 15 people visiting Amazon bought something, for example, that would be a 6% conversion rate. This 20% difference means that "in Q1 2014 an iPhone user was 25% more likely to make a purchase than an Android phone user." And there was a similar gap in the amount spent per session - a 15% difference. This equates to an iPhone user spending an average of $1.18 for every dollar an Android user spent.

A year later, however, and that gap is rapidly shrinking.

In Q1 of 2015, there was only a 5% difference in conversion rate, meaning an iPhone user "is only 6% more likely to buy."

And Android users are also spending more - comparative to iPhone users - than ever before. There is now only a 9% difference in revenue per session, meaning that an iPhone now user spends just $1.10 for every dollar spent by an Android user.

• Gap in spend rates favouring iOS over Android:
• Q1 2014: 15%
• Q1 2015: 9%

When it comes to certain kinds of devices, Moovweb found that some Android models had even higher conversion rates than the current-generation iPhone 6 and 6 Plus. Those included the Moto X, the HTC One M8, and the Droid Razr.

This isn't happening everywhere (but it is happening where it matters)

Of course, this isn't a global trend. The data was drawn from "traffic to websites of North American retailers whose mobile websites we power," Moovweb employee Anis Salvesen clarified to Business Insider. Obviously, in emerging markets like China and India where low-end Android handsets are proliferating, there will be a wild disparity in conversation rate and amount spent per session. And as the next billion come online, the gap might grow larger, not shrink.

But it's still a surprising shift, and suggests Apple can no longer rely on its userbase's wealth in mature markets to give it the edge over Android.

And it's a shift that is also being repeated in other metrics. In a recent blog post, Andreessen Horowitz analyst Benedict Evans notes that an average Android user spends half as much an average iPhone user does. It's a big difference, sure - but a year ago, Android users were only spending a quarter as much. Evans speculates that slowed growth of the Apple App Store revenue may indicate that people are spending less on average" - making it easier for Android users to catch up.

He also notes that Google Play is now twice as big as the App Store (50 billion downloads every 12 months for Google vs. 25 billion for Apple); and that App Store revenue seems to have stalled while Google Play revenue, although lower, is still growing.

Apple is losing its ace in the hole

The premium spending power of its userbase has always been iOS's trump card, negating Android's vastly larger number of installs. It proved to advertisers and developers that it was the "stronger" device - and lent it aspirational value with consumers too.

But if the purchasing power of Android users continues to grow, and the gap continues to shrink, this may not be the case for very much longer.

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