1 3 Computing System Fundamentals 3.6 Errors

3.6.2 Prevention and Recovery

3 Methods Verification - checking the data on the source document are Exactly the same as those being entered. Validation - checking the data input into the system are Acceptable (make sense/are possible).

4 Verification Is it exact? Is it equal? Is it accurate? Verification has to involve humans.

5 Verification methods Visual verification i.e. proof-reading. Double entry verification - the data from the source document are entered twice, the keyboard may lock and the computer sound an alarm if the second item is not the same as the first.

6 Validation Is it reasonable? Is it possible? Is it acceptable? Computers can be programmed to validate.

7 Validation methods Presence check - some fields are required (must contain data) e.g. customer account number on a sales order. Range check - do the data fall within a certain range e.g. an age outside the range 0 to 120 cannot be entered.

8 Validation methods Character count e.g. a French postcode must have 5 characters. Format check e.g. a UK postcode has the pattern TTN NTT, where T is a letter and N is a number.

9 Check digits A digit calculated from a data item number, then tagged onto that number and entered into the computer, the latter can then calculate back to validate it.

10 Check digit example Product code: Add the digits: = 41. Divide by 10: 41 div 10 = 4 mod 1 So use 1 as the check digit, the product code becomes

11 Parity bits This system checks if a byte is complete, typically during data transmission. There are 128 ASCII characters, so only 7 bits of a byte are needed, the 8th bit can be used as the parity bit.

12 Parity bits Under even parity, the parity bit is set so there are an even number of 1's in the byte ‣ would get the parity bit 0 and ‣ would get a 1. The receiving device checks each byte for correct parity.

13 Parity bits If the system is using odd parity, there must be an odd number of 1’s. The sender and the receiver must both be using the same parity (odd or even) - this is part of the network protocol.

14 Batch and control totals With groups of records, a batch total (number of records) and control total (sum of a particular field from each record) may be added to the file when the batch of records is transmitted to ensure that all are received correctly.

15 Check sums When software is transmitted, a check sum is generated (the sum of all the instructions when treated as numbers), which gets checked before the software is executed.

16 Online processes For a data entry error in an online process, re- inputting the data is usually the most efficient method of recovery.

17 Batch processes If it is a batch process, the errors will be reported (printed out) and the batch process will continue with the next input. The entries with errors can then be corrected and included with the next batch.

18 Real-time processes With real-time processing, inputs have to be verified instantaneously - this can cause problems for control systems.

19 Transmission errors Transmission errors can be caught by parity bits and/or check sums can be automatically corrected in some circumstances but more commonly there will be a request for a re- transmission of the data.

20 Backups Many organizations rely heavily upon computer-stored data so it must be protected. Backups are taken every day. Backup media should be stored in a fireproof safe or at a different location.

21 Backups They are often rotated over 2 weeks but can be kept for months or years. Full backups save a new copy of every file. Incremental backups save only those files which are new or have changed since the last backup.

22 Backups Archiving backs up then removes the old files from the system. Magnetic tape is a common backup medium because it is cheap and data won't need to be accessed directly, only sequentially (it is accepted that restoring a backup will not be a fast process).

23 Deliberate errors These are hardest to recover from - the offending user usually knows enough to get around any validation checks. File access permissions should be imposed by the operating system and system administrator.

24 Deliberate errors They require vigilance from the administrator. Cross-referencing data to other reliable sources will show if deliberate damage has been done.