What Size Magnet Can ERASE a Credit Card?

Something I would like to point out about the Square stripe readers is that Square's instructions tell the merchant to make sure that the phone or tablet's volume is turned all the way up so that the device has the greatest sensitivity for capturing the signal through the headphones port.  It also states that one should swipe the card SLOWLY.  Running the card through the reader as quickly as the guy at the Farmer's Market did would cause the card not to read even when in pristine condition.  When one swipes a card, it should take most, if not all, of a second to make just a single pass.  Any faster than that and the reader will report a bad read more often than not, especially when swiping back and forth five or six times in a single second as seen in this video.

Both of these issues could have affected the results of this experiment, so I would like to suggest that if Dianna et al. would like to test for more accurate results, take both of these factors into account when repeating the experiment.  If the only desire for this experiment was to test whether magnets could affect magnetic stripes on cards, then no need to re-run this experiment since it already proved that point despite the procedural errors.