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Grade Grinder

Timestamps


When a file is first created in Fitch, Tarski's World, or Boole, it is given a timestamp. The timestamp encodes the exact millisecond the file was created. In addition, each time that file is opened or saved by one of the LPL applications, another timestamp is appended. So the file contains a record of the times when edits that have been made to it. Every saved file contains a minimum of two timestamps-- more, if the file has been opened and saved several times.

It is extremely unlikely that two files would be created in the same millisecond, and even less likely that they would then be saved at exactly the same time. So if two files have the same timestamps, then they are almost certainly copies of the same file. (Since each timestamp is a 12 digit number, having a coincidental match is about as likely as winning the California lottery.) Whenever a file is submitted to an instructor, we record its timestamp information in a database. We also check that information against all other entries in our database. If an entry is found that begins with the same timestamp, then that file has already been submitted. We then check the bookID of the person who submitted the first file. If the bookID's are the same, then this user is just submitting the file a second time. If the bookID's are different, however, then there are two users who have submitted copies of the same file to an instructor.

In this case, we have a "timestamp collision." We then include in the grade report a message indicating that a collision occurred, pointing to this web page for information.

There are two types of timestamp collisions that we report. If two files have identical timestamps, then this means all of the modification times match, and that the students (most likely) submitted exactly the same files. In this case we report that the solution files are "identical."

It is possible, however, for two files to start out with the same timestamps, but then to be separately modified and saved. In this case a file will have at least two matching timestamps, but then will have two or more different timestamps. This will happen if a student copies another student's file, but then opens and saves it on his or her own disk, perhaps making modifications in the process. In such cases we report that the student has turned in a file that is "similar" to an earlier submission. It is overwhelmingly likely that the two started out with identical files, but then went on to modify them separately.

Please note: The presence of a timestamp collision in a student's grade report should not be taken as evidence that this student has obtained a file from elsewhere. We report collisions on the second and subsequent submissions of a file, so this is evidence only that this file has been submitted before. A student, for example, might complete a file and go on to work on other exercises. An evil-doer might copy and submit the file, with no resulting collision report, while the first student completes the other work. When the original student submits the file, a collision report will appear in their grade report, even though they are blameless.

We provide reports of timestamp collisions solely for your information, to help you enforce whatever policies you set in your class. If you have a policy that allows students to work together, then you may want to ignore these messages. If you ask students not to work together, then you can decide how to handle reported collisions.