Verification and Validation


Background:

In this section you should be able to:

 


Introduction


Data integrity


Validation

Validation techniques include:

 

Range check Only accepts data within a specified range
Presence check Rejects blank fields
Length check Only accepts data of a specified length (often not over a certain number of characters)
Type check Only accepts the correct type of data, e.g. numeric
Format check Only accepts data in the correct format or pattern, e.g. UK post codes which are normally 2 letters, 2 or 3 numbers, 2 letter
Look-up check Data matches limited number of entries from a list
Intergity or Consistency check Data is compared to or checked with other known data, e.g. "Miss" should be "female"

 


Verification

Verification Techniques include:

 

Double-entry verification Data is entered twice by different people and compared for discrepencies
Screen verification After being entered, data is displayed on screen to be checked
Check Digit Extra digit at the end of a number that is produced from a standard calculation, if the data is correct then the same digit is produced, e.g. barcodes, ISBNs
Batch totals

Before a batch of data is entered, one or more fields is added up and entered with the data. When the data has been entered, the same figures are totalled and compared with the batch total - they should match

Control totals Batch totals that have a meaninful value, e.g. total value of orders entered
Hash total Batch totals that have no meaning, e.g adding up the dates of the orders

 

Verification techniques used in data communications:

Parity check An extra bit added to indicate how many 1s make up the ASCII code. This is compared when the data is received and may indicate an error in the data.
Checksums Extra data that is worked out from a block of data and sent with the data. When the data is received, the calculation is repeated and compared with the sent checksum.
Cyclic redundancey checks (CRC) Specific type of checksum technique used for data stored on optical disks or transmitted across a network

 


Exercise 1:

  1. Quiz - Validation or verification?
  2. Take notes from Verification & Validation - Presentation (Payne-Gallway Publications)

Exercise 2:

  1. Complete this exercise sheet by working through the web site What is validation?
  2. Revision exercise
  3. Complete this worksheet that accompanies the text book

Data Transmission Error Checking

Parity Bit

Character

7-bit code

Even parity

Odd parity

a

1100001

1 1100001

0 1100001

There is an odd number of 1s

To make the parity even, a 1 is added to the left

To make the parity odd, a 0 is added to the left

q

1110001

0 1110001

1 1110001

There is an even number of 1s

To make the parity even, a 0 is added to the left

To make the parity odd, a 1 is added to the left

 

Drawback with parity bits:

Sender

Receiver

10011001

Parity is even, but there is a 1-bit corruption

10011000

There is an odd number of 1s (instead of an even number), so an error is detected.

11000011

Parity is even, but there is a 2-bit corruption

01000001

There is an even number of 1s, so there error is not detected.

 

Exericse 3

Error Checking: Block Parity Checking

 

Errors can happen when data is transmitted from one computer to another.

bit number

7

Row

parity

bit

6

5

4

3

2

1

0

1st byte

0

1

0

0

1

0

1

1

2nd byte

0

0

1

0

1

1

1

0

3rd byte

0

1

0

1

1

1

1

1

4th byte

1

1

0

1

0

1

0

0

5th byte

0

1

1

1

0

1

0

0

6th byte

1

1

0

1

0

1

1

1

7th byte

0

1

0

0

0

0

0

1

8th byte

1

1

0

0

1

0

0

1

9th byte

1

1

0

1

0

1

0

0

10th byte

1

0

0

1

0

1

0

1

11th byte

0

1

0

1

0

0

1

1

column

parity bit

1

1

0

1

0

1

1

1

 


Complete overview


Exericse 4



Now do the questions at the end of Chapter 16 of the text book