OptoFidelity said that it used its own Touch Panel Performance Tester to measure the iPhone 5C, iPhone 5S, and Samsung Galaxy S3. The device uses a robot to "touch" exact coordinates on a screen. It then measures the coordinates of the touch device to see how closely they matched.
According to the data (PDF), the iPhone 5S and iPhone 5C had significant fail rates, especially around the edges and toward the top of their displays. The Samsung Galaxy S3, meanwhile, was largely accurate across the middle of the screen, but had errors around the edges.
From there, OptoFidelity turned its attention to the virtual keyboard on the three devices and again found that Samsung's device is far more accurate in its response to touch inputs.
"Because the touch accuracy is more even in Galaxy S3 than in iPhone 5S and iPhone 5C, you get a lot less typing errors, and letters which are close to the edge are working better," OptoFidelity said in its report. "You can also test this by tapping letters on the edge and typing long text messages."
OptoFidelity didn't say why it didn't include the Galaxy S4 in its study.