2. Data must be carefully control becauseTo get accurate and meaningful result
To protect the computer hardware from being damaged
3. Sources of error (錯誤來源)Fill in error (填寫錯誤)
Transcription error (抄寫錯誤)
Input error (輸入錯誤)
4. How to control? Data verification (數據核對)
Data validation (數據驗證)
5. Data verification (數據核對)Display data to let the user check again and ask for confirm
Double check
Triple check
…….
For example : Bill Payment System, Ticket Booking System
6. Data validation (數據驗證)Programmer foresee possible errors and detect them and probably correct them
Presence check - This field must be filled?
Range check - Data exceed a limit?
Type check - Data type match or not?
Consistency check - Strange data appear?
Control total - total no. of record correct?
7. Data validation (數據驗證)Check digit
Can be found in ID card, bar codes, ISBN …
Use formula to check the data is correct or not
For example : this student ID 1234(0) correct?
Formula : Add all digit, Divide the sum by 10, Found the remainder
In this case, this is a valid (有效) ID !!
8. Common used checking methodWeighted modulo 11 check
Formula :
unit digit X 2
tenth digit X 3
hundredth digit X 4
:
add all, mod 11, use 11 - the remainder
9. HK ID. No. CheckingA237637(A)X 2X 3X 4X 5X 68141835 14 = 122122 mod 11 = 1check digit = 11 - 1 = 10X 7X 8249++++++It is valid有效!!
10. HK ID. No. CheckingZ297138(0)X 2X 3X 4X 5X 620814543516= 340340 mod 11 = 10check digit = 11 - 10 = 1X 7X 849++++++It is invalid無效!!