RNJ 10/02/091 3 Computing System Fundamentals 3.6 Errors Prevention and Detection

RNJ 10/02/092 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). Methods

RNJ 10/02/093 is it exact? is it equal? is it accurate? has to involve humans. Verification

RNJ 10/02/094 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. Verification methods

RNJ 10/02/095 is it reasonable? is it possible? is it acceptable? computers can be programmed to validate. Validation

RNJ 10/02/096 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 Validation methods

RNJ 10/02/097 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. Validation methods

RNJ 10/02/098 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. Check digits

RNJ 10/02/099 Product code: Add digits: = 41 Divide by 10: 41 div 10 = 4 mod 1 Use 1 as the check digit, product code becomes Check digits - example

RNJ 10/02/0910 Checks if a byte is complete, typically during data transmission 128 ASCII characters, so only 7 bits of a byte are needed, the 8th bit can be used as the parity bit Parity

RNJ 10/02/0911 Under even parity, the parity bit is set so there are an even number of 1's in the byte ( would have the parity bit 0 and would have 1) ‏ Receiving device checks each byte for correct parity Parity

RNJ 10/02/0912 If the system is using odd parity, must be an odd number of 1's Sender and the receiver must both be using the same parity (odd or even) - this is part of the network protocol. Parity

RNJ 10/02/0913 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. Batch and control totals

RNJ 10/02/0914 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. Check sum