Objectives of Control

The objectives of control are:

To ensure that all data are processed To preserve the integrity of maintained data To detect, correct and re-process all errors To prevent and detect fraud

Types of Controls

The different controls can be grouped under five headings:

Manual controlsData protection controlsValidation checksBatch controlsOther controls

Types of Error

System designers must guard against the following types of error:

Missing source documentsSource documents on which entries are omitted, illegible or dubiousTranscription errorsData preparation errorsProgram faultsMachine Hardware faults

Manual Checks

Even in advanced systems checking of source documents is necessary. Checks may include:

Scrutiny to detect: Missing entries Illegible entries Illogical or unlikely entries

Reference of the document to stored data to verify entriesRe-calculating to check calculations made on the document

Data Collection Controls

The collection of data for processing involves transcribing it into a form suitable for machine processing.

There is a real possibility of error at this stage. Controls must be imposed to prevent or detect

errors at this stage. The type of control depends on the method of data

collection used:

Data Collection Controls 2

On-line systems. These depend on the data displayed on a VDU or printed being checked by the operator before being processed.

Character recognition. With these techniques, accuracy depends on the character reader detecting any doubtful character or mark.

Validation Checks 1

A computer can’t notice errors in data being processed in the same way that a human operator can.

Validation checks are an attempt to build into computer programs the ability to detect and report incorrect data items

Checks can be made at two stages: Input – when data is entered Updating – after processing

Validation Checks 2

The main types of validation check used are:

Presence. Data are checked to ensure that all necessary fields are present e.g. A payroll program must have complete employee and national insurance fields.

Size. Fields are checked to ensure they contain the correct number of characters e.g. Student_Id should contain 6 characters i.e. G01234

Range. Numbers or codes are checked to ensure that fall within a permissible range e.g. DOB in a school database should fall between 1982 and 1989.

Validation Checks 3

Character. Fields are checked to ensure that they contain only characters of the correct type e.g. there are no letters in a number field.

Format. Also called a picture check. Fields are checked to ensure that the format is correct e.g. that a code contains the correct number of letters and numbers AND they are in the correct sequence.

Reasonableness. Quantities are checked to ensure that they are not abnormally high or low.

Check Digits. See next slide.

Check Digits

Use of a check digit allows a number to be self- checking.

It is calculated using a mathematical formula and then becomes part of the number itself.

When the number is input into the computer the validation program uses the same mathematical formula to check the number and ensure that the number is correct i.e no digits have been transposed.

The modulus 11 algorithm is used to create check digits for ISBN numbers.

Modulus 11

Assign each digit a weight. The right hand digit (the least significant) is given a weight of 2, the next digit to the left 3 and so on

Multiply each digit by its weight and add the products together

Divide the total by 11 and find the remainder

Number 2 5 4 6

Weight 5 4 3 2

Products 10 20 12 12

Total 54

Modulus 11 - 2

54 divided by 11 = 4 remainder 10. Subtract the remainder from 11 to find the check

digit. 11 – 10 =1 The new number is 25461

Now try this. The ISBN number of Pat Heathcote’s book is 095324900. What is the check digit?

Why is it presented in this format?

Batch Processing 1

Batch controls are fundamental to most computer based accounting systems. The main stages of batch control are:Batching. Documents are arranged in batches by being placed in a wallet or clipped together. A batch cover note (see next slide) is attached to the batch.Numbering. Each batch is allocated a unique number, which is entered on the batch cover note.Batch Registers. Each department responsible for processing the batch records its receipt and dispatch in a register. It is then possible to check that all batches have been dealt with and trace any batch that gets lost or delayed.

Batch Processing 2

Batch Totals. Control totals are obtained for each batch. The control totals comprise: The total number of documents in the batch Totals of the fields that are required to control e.g.

total value of invoices Hash Totals. A hash total is a sum of values calculated

solely for validation purposes e.g. adding all the employee numbers together.The computer should be able to perform the same calculation and report any discrepancy.

All of the above is recorded on the batch cover note so that the results of processing can be checked against manually calculated values.

Verification

Verification is the process of entering data twice. The second entry is compared to the first to

ensure that it is accurate. It is commonly used in batch processing where a

second data entry clerk will key in each batch to verify it.

You will have come across this technique when changing a password and you are asked to re-confirm your new password.

Validity vs. Accuracy

It is possible to to ensure that data is valid. It can never be guaranteed to give us correct

information as the following examples show: “A student’s DOB is 02/01/87”. The data falls within a

valid range and is entered in the correct format. However, her actual DOB is 02/10/87!

“The market research questionnaire data we collected shows that 95% of the population eats soup twice a day”. However, this research was carried out outside a soup kitchen!

Remember valid data can lead to inaccurate information.

Further Reading

Paper Heathcote – pages 127 – 131 De Watteville – page 164 Mott – pages 66 – 68

Web www.creditron.com/checkdig.htm

(Bar codes and Mod10 explained) www.lcweb.loc.gov/issn/check.html

(Modulus 11 explained)