To pass, the variation cannot exceed 0.6 seconds per day. This figure is obtained by subtracting the average mean daily rate of the first two days of testing from the mean daily rate of the last test day. To pass, the resumption of rate can be no more than 5 seconds. The COSC certificate is not a precise indicator of how well a watch will perform on the owner's wrist. Rather, the test results show how well a given movement performed over a given period of time under certain conditions.
Several factors can affect the actual performance the owner observes.
First, The COSC test is only an artificial approximation of actual use. Movements can spend significant time in the vault. When they do, they are usually disassembled, cleaned, re-assembled, oiled and regulated, then cased. This can certainly affect the movement's performance, yet re-testing is not required. Next, the watch may be affected by shipping in the distribution chain, where it might be exposed to shocks or extreme temperatures.