This is an interesting question and shows a fatal lack of understand about lean. It shows a lack of understanding of how a system works. A system is only as strong as its weakest link. Sub optimize one and your sub optimize the whole system. If you favor one over the other you will weaken whatever initiative exists. You have to have a balanced perspective.
These two aspects of lean implementation are critical but only if they create customer satisfaction and personal development of those employees involved. The processes can be easily defined which can be the initial measure of your transformation's success. The process definition allows anyone to examine its content and find waste. This process-related step is the initial validation of lean.
People initiate action, develop logical ways forward and solve the problems of having waste in the system. People are the engine of change. I can’t imagine having only one of these elements to achieve lean. They both must be present. This does not mean that you have to work on both simultaneously. Placing emphasis on either process or people can initiate or develop change and can be a first step. But failure to integrate the other will create a system that will fail.
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