Data Validation

Data validation – is the computerised checking of input data for errors (data that may be unreasonable or incomplete) before it is processed. The method of validation used is depends upon what data is being entered.

Range check – ensures that the data entered is within a certain range. E.g. when you enter a number corresponding to a month of the year, the range of acceptable numbers is 1 – 12 (maximum of 12 months in a year).

Data type checks – this is also known as character or alphanumeric check. When a database is created each field in the database is given a type. Whenever data is entered into a field the database will check that it is of the correct type, if it is not, then an error message will be displayed and the data will have to be re-entered. E.g If a number is to be entered and you enter text.

Inconsistency/Consistency check – this checks one piece of data against another. E.g. the data may include both gender [M or F] and title [Mr, Mrs, Miss]. If someone has entered M and Mrs, then the two data items are inconsistent.

Data Verification

Data Verification – is the checking for mistakes such as transcript errors when data is copied form one medium or device to another. This process is carried out by humans but it does not guarantee that the entered data is correct.

Errors may occur when employees are keying in data from a source document. Types of errors:
Typographical errors – are the typing errors that we all make when we hit the wrong key. E.g. I sent down the street and saw my friends playing cricket.

Transposition errors – happen when we enter numbers or characters in the wrong order. I wnet down the street and was my friends playing cricket.

Data verification is used to reduce these types of errors. Visual verification is not efficient for large amounts of data. So to verify data:

Double entry verification – is when the data is keyed in a second time, and the first entry is checked against the second. A warning message is displayed if the entries do not match. The employee then has to check whether the error was made in the first entry or the second and confirm which is correct.

Describe various methods of file organization and access;

Sequential field ordering – describes a file that is sorted using the data in one of the fields. In sequential file all the records are sorted in order. One of the fields will be used for the ordering, that is, sorting the file. This field is known as the sort key. Sequential files are searched using sequential access. That means that if you want to find a record in a sequential file you have to start at the beginning and work your way through until you find it.

Random – uses a formula to find the position of a record on a direct access medium. It is the most efficient way of storing extremely large files. Each position will have its own specific address on the disk. The position allocated to each record is calculated by a special formula. This formula will use some of the data in the record then convert in into the address of a position on the disk. When someone wants to search for a record, once again the formula will work out where it is.

Index-sequential – uses an index file to speed up searches on a sequential file. This makes it possible to search large sequential files very quickly. It can not be used on sequential access media but can only be used with direct access media. The main sequential file containing all the records could be stored on a hard disk (which is a random access device). Each position is given a numerical address. The index file will also be stored on the hard disk alongside the main file of records. In the index file the position field will hold the address of a position on the disk.

7. select appropriate file organization for particular application

Associate an appropriate file structure and access method to a specific application. For example, a payroll file would be organized sequentially with sequential access.
A payroll is a list of all the employees in an organisation, together with their pay details. Every month a payroll application will calculate how much each person should be paid. It will print payslips and it may send the details of payment s directly to the company’s bank. The payroll file would normally be stored sequentially by surname. At the end of each month the employee’s record is accessed one at a time. So a ___________ file is needed

A supermarket uses a computerised stock system. The stock records hold details of the items for sale in the store. Each item is identified by a bar code. The stock record for each item also holds the price of the item and the number in stock. When a customer wants to buy an item the cashier scans the barcode. The stock system then finds the record for the item and sends the price to the POS terminal. It also updates the stock record to show that the number in stock has gone down. There will be a large number of records in the stock file and each item has to be retrieved quickly, so the best way to organise the file would be as a ____________ file.

A library has a computer-based catalogue of all its books. The catalogue is sorted by ISBN which can be found on the back of any book. There is usually a barcode that gives the ISBN as well. When someone borrows a book the librarian scans the ISBN barcode and this brings up the details of the book on screen. The catalogue file is large, but stored sequentially. __________________ file organisation will give fairly quick access to any record.

QUESTIONS

1.What is a turnaround document?
2.What are the advantages of using machine-readable documents?
3.Why do forms sometimes make you write each character in a separate box?
4.What is the main difference between data verification and data validation?
5.Why is it better to use double entry for data verification rather than simply checking over what has been entered?
6.Explain the difference between sequential and direct file access.
7.What is a sort key?
8.Give an example of a file that would be organised sequentially?
9.What is the role of an index file in index-sequential file organisation?
10.Of the three methods of file organisation which, if any
a.Can only be used with direct access media?
b.Can be used with both direct access and sequential media?
c.Can only be used with sequential media?