Buying used phones might seem like a risky move. After all, who knows what the previous owner did (or didn’t do) when they were using it?

However, a company called OptoFidelity — which automates smartphone testing for various companies — just released some data it’s discovered about used phones. It turns out that less than four percent of used phones submitted to OptoFidelity for testing come up with a problem — and its sample size is huge, nearly one million devices.

What’s more, OptoFidelity breaks down just which failures come up the most when testing these used smartphones. You can see the chart below, but the general takeaway is this: the feature that fails the most is the physical buttons, with the second-most-likely-to-fail feature being the display.

Here’s the chart:

OptoFidelity

When you think about this, it makes a lot of sense. The physical buttons only can be pressed so many times before breaking and the LED display can only be lit for so long before having problems.

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Frankly though, we’re surprised connectors don’t fail as much as the speaker, considering there are probably a lot of people jamming broken and/or wrong cables into ports all the time. Although, there are probably also a lot of folks who crank their speaker loudness to the max, which puts some stress on the hardware (a ride on any city bus will attest to this).

OptoFidelity points out that this small percentage of failures it records in smartphones are in models that either never make it to the used market or are repaired before their second-hand sale. The company also reminds shoppers that the best way to buy used phones is to only buy from reputable sellers, such as big box stores like Best Buy or eBay/Swappa sellers who have positive feedback.

If you want to know more about buying used phones, check out our guide below!

NEXT: Why you should (and why you shouldn’t) buy a used smartphone