Data Control

F.4 C.I.T. Data Control

Data Control Check if the input data is correct  Why? 

 Garbage-In-Garbage-Out Input Error Input: Gender = F

Process Count: no. of M no. of F

(GIGO) Output Error Output: Male = 29 Female = 1

Error 

Sources of Error  Data Source Error  Incorrect data are provided 

e.g. Missing data field, like the age

 Transcription Error  Data is read or typed wrongly 

e.g. “1” and “l”, “u” and “n”, “0” and “O”

Errors caused by manual input

 Transposition Error  Swapping of two characters 

e.g. “bye” as “bey”

 Transmission Error  Errors while sending data from one source to another 

e.g. “10010101” instead of “10000101”

Error 

Consider the following test paper.  What’s

wrong?  Why wrong?

Data Control 

Data Validation  Check



if the input data is reasonable and valid

Data Verification  Check

if input data is same as in source document

Data Validation 

Range Check  E.g.

Class no. of 4A students: 1 – 30  E.g. Error input: 40 

Presence Check A

particular field must have data  E.g. Missed date 

Length Check  E.g.

Telephone number: 8 digits  Error input: 3567890

Data Validation 

Format Check  E.g.

Class: [1-5] + [“A”-“E”] or [6-7] + [“A”-“D”]  E.g. Error input: “8G” 

Control total  Sum

of numeric fields  E.g. Sum of all class numbers Manually calculated check sum Compare Accumulate the field

Further Process

Data Validation 

Check Digit  E.g.

HKID no., ISBN, etc  Self-checking Data itself can prove its validity  E.g. ID card number: 

Leftmost alphabet:  A = 1, B = 2, C = 3, ……, X = 24, Y = 25, Z = 26  A 1 2 3 4 5 6 (3) x Weight: 8 7 6 5 4 3 2 1  Weighted Sum =8+ 7+ 12+15+16+15+12+ 3 = 88  Is weighted sum divisible by 11?  Yes: Valid ID 

Data Validation 

Check Digit  Self-checking

Data itself can prove its validity  E.g. ID card number: 

A 6 5 4 3 2 1 (3) x 8 7 6 5 4 3 2 1  Weighted Sum =8+ 42+30+20+13+6+ 2 + 3 = 123  Is weighted sum divisible by 11?  No: Invalid ID 

The ID number can tell its validity!  Now, check your ID card no. 

Data Validation 

Check Digit  How 

to find?

E.g. K434925(?) (11) K 4 3 4 9 2 5 (?) x 8 7 6 5 4 3 2 1  Weighted Sum =88+28+18+20+36+6+ 10 = 206  Since 206 + 3 = 209 which is divisible by 11  Check digit = 3  Another way: 206 / 11 = 18 ……8  Check digit = 11 – 8 = 3 

Data Validation 

Check Digit  11

is used as the divisor to test the validity

It is called “weighted modulo-11 algorithm”  Also applied in ISBN 

Data Verification 

Compare input data with source document  Proofread

the screen before process  Display the input data for final checking and confirmation  Input data twice  

Check if the inputs in different time are same E.g. Especially important for password confirmation 

Because password will not be shown for confirmation!

 Double 

entry

Check if the inputs by two different people are same