What kind of changes should I make for process audit for the internal audit schedule and plan for ISO 9001:2000.
What will be the difference between system audit and Process audit ? What extra I have to do to audit 'Purchasing' as per Process audit?

For the system audit, you just need to follow the ISO requirement, check whether it fulfill the requirement. For the process audit, it is more details, and you have to go into "how it works".

For example. To audit the system on the purchasing, you need to check whether the input and output are same, all the record is able to provide trace. But for the process, you need to audit how they control on the input and output, not only documentation but the actual practice

What is the difference betveen Process and System

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ISO 9000 (http://www.qmsconsultants.com/ISO-9000%20ISO-9001.html)