More structure can help here. My fave example: 9999 9999 9999 9999 9999 9999 9999 9999 9999 Now the users will not always pick the beginning or th

More structure can help here. My fave example: 9999 9999 9999 9999 9999 9999 9999 9999 9999 Now the users will not always pick the beginning or the end when they fail to compare the whole thing. That is because they can easily identity each group to the other person (combinations of top, middle, bottom, left, right, centre).