Checking data Chapter 7 Prepared by:Sir Mazhar Javed

Consequences of errors Customers becomes angry if they are sent bills that are too large and companies lose money if bills are too small. Wrong decisions are made by the managers because the decisions are based on wrong information A company could be prosecuted under the Data protection Act 1998 if the incorrect information causes the person concerned some kind of loss

How are errors avoided Verification Checking the data which has been copied from one medium to another to see that it still represents the original data. Checking that what is on the input document is exactly the same as what is entered into computer Examples : When data is encoded into disc, a keyboard operator reads the data from a source document. The data is then verified by a second operator, who retypes it all. A magnetic tape cassette can be used to store the contents of parts of a computer’s main store. If the tape is rewound the computer can read what is recorded and verify that the contents have been saved correctly.

Methods 1.Double entry: Two people trying in the same data and only if the data is identical is it accepted for further processing Problem: two people need to be paid for doing the same job. They make identical errors so the mistake still goes unnoticed. 2.Proof reading: carefully checking what has been type in against what was on the original document

Validation Checking of data before the main processing to see that it is acceptable for the process Is the process of detecting any data that is inaccurate, incomplete or unreasonable Performed by a computer program. Validations programs will usually perform some or all of the following Types of checks. Character type checks : The right type of characters have been entered. such checks detects numbers where characters should have been entered and vice versa Range checks: Performed on numbers to make sure that lie specified range

Validations checks types Presence Check: When entering data in database the completion of some fields may be optional. There are certain fields which must contain data and the system will not allow them to remain blank. E.g National Insurance Number in a payroll system Hash Totals: A hash total is meaningless total Control Total : Like a hash total except that the total has some meaning.

Validation checks Batch header check: The total number of records in the batch should be calculated by the computer and compared with the figure on the batch header. The control total and hash total are also calculated and compared. Spelling checkers: Spelling checkers are not just used in word processing software. Any software where accurate text needs to be entered can have a spell checker facility. Format Check: the code perhaps has a pattern of 2 letters followed by 4 numbers. The quantity and price must be numeric. Lookup Tables: Stock items are given a unique code to identify them. Whenever this code is used, it is checked against a table stored by the computer to make sure that it is valid stock code. If the code is not in the table it is rejected, thus preventing any processing using incorrect codes.

Validation check type Check digits: When any large number is input to a computer, either using a keyboard or using a direct input device such as a barcode reader, there is always a chance of errors. These large numbers are important, it could be product number, employee number or account number. In this additional number is included, placed at the end of the original number. This additional number is called a check digit and is calculated from the other number Example : when the account number is input the computer cuts off the last digit which is the check digit, it then uses the rest of the numbers to recalculate the check digit, which it then compares with the removed digit. If the number are the same then the account number has been input correctly.

Validation checks Custom dictionaries: Despite having a huge dictionary, most word processors still fail to recognize certain words. There are two types Proper names – names of person and places Specialist word used in a narrow field, medical terms, legal terms. Length checks: Sometimes a certain item of information is always of a certain length: it contain a set number of characters. In a national insurance number ha more or fewer characters than this, then the length check will alert you to the fact that it has been entered incorrectly.

Validation Checks Parity errors: When data is passed along a communication line it is important that the data is not corrupted in any way and if it is. The checking of data after it has passed along a communication line is performed by using a parity bit. The computer adds up the number of bits in one byte and if the parity is different to the parity setting the computer will report an error. Use either even or odd parity. Example: we sending the letter c then it bits in ASCII is In this there is three 1s in this code and odd parity is used, if we add 0 bit in left side then it becomes odd parity and if 1 is added on left then it becomes even parity. How Modem work: modem have a chip inside them to deal with parity checks,the sending modems adds the parity bits and receving unit calculates what parity bit should be. If error occurred transmission parity will no longer is observed checks is that Problem: if more than one error occurs, parity can still appear to correct

Types of errors Transcription errors: Transcription errors occur due to misreading or mistyping data. This may be caused by bad handwriting or confusing, for instance, the number 5 with the letter S or 0 with O. Transposition errors: Occurs when two digits or letters are swapped around. If you are typing in data at high speed you don not always look at the screen and it is very common to end up with say ‘ot’ instead “to” or 5124 instead of it is estimated that about 70% of all errors are transposition errors It is estimated that about 70% of all errors are transposition errors.

How to avoid errors Despite all the checks that we can perform on the ‘raw’ data before it is accepted into a system some errors are bound to occur. If data is entered into a system via a keyboard, the simplest way of reducing errors is for the person who is entering the data to proof read it carefully against the original document

Checking of error in word processing There are two situations to consider Where the typist has been given a copy of a document or where a document is on an audio tape. In this situation a typist will be working at high speed and will be looking at the document or listening the tape. The spell checking facility on the word processor can then be used. The second situation is where someone types their own document This is much slower process. A slow typist will probably correct any typing mistakes as the text is typed. The document still need spell checking for proof reading.