To test altimeter accuracy, we calibrated each watch at a common location, hiked uphill, noted the altitude at the summit, then hiked back down again. By doing this we were able to determine which provided the most accurate readings. We did this several times in a wide range of weather.
To test temperature, we placed each watch on a non-conductive surface to see how their readings compared to that of a classic mercury thermometer. We also performed the same test while wearing the watches on our wrist for 10 minutes. Battery life was tested via field tests, online surveys, and at-home tests to determine which watches lasted the longest (with and without GPS function), and which ones didn't last hardly any time at all. Some watches (with a battery) couldn't be tested as their estimated battery life was three months (and our testing period is three months). We also compared GPS function between each GPS watch and determined which had the most accurate GPS over many days of running the exact same route and comparing tracks.
We even took the GPS watches into the depths of the Earth (where GPS reception is poor) and determined which watches were best able to keep a GPS lock and provide accurate distance, pace, and altimeter information. To determine the best watch display, we compared reflectiveness in sun (with and without sunglasses), and also considered the length and brightness of each watch's night light. Finally, for ease of use we simply looked at how easy the watch is to figure out without consulting the users manual.
After performing all these tests, we rated each watch using six different metrics: features, ease-of-use, battery life, altimeter accuracy, display quality, and comfort and fit. After all these in-field and at-home tests, we have produced (what we think) is the best altimeter watch review out there.