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3 Computing System Fundamentals

3.6 Errors

3.6.2 Prevention and Detection

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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

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is it exact? is it equal? is it accurate? has to involve humans.

Verification

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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

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is it reasonable? is it possible? is it acceptable? computers can be programmed to

validate.

Validation

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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

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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

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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

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Product code: 0198327633 Add digits: 0 + 1 + 9 + 8 + 3 + 2 +

7 + 6 + 3 + 3 = 41 Divide by 10: 41 div 10 = 4 mod 1 Use 1 as the check digit, product

code becomes 01983276331

Check digits - example

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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

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Under even parity, the parity bit is set so there are an even number of 1's in the byte (0010100 would have the parity bit 0 and 1101011 would have 1)

Receiving device checks each byte for correct parity

Parity

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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

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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

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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