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Is human resource planning only relevant to large companies or should small businesses do HR planning too?
Many people associate human resource planning with what very large companies do -- IBM, or Ford. That's because, almost by necessity, large companies need to have a much more formal and comprehensive approach to HR planning because of their size and the complexity of their businesses.
That said, even a business owner with a very few employees need to think (that is, plan) about various personnel and human resources issues. Many small business owners do this without really thinking about it. For example, a small business owner needs to think and plan about what benefits to offer, how to manage growth of staff, how to plan how many staff are needed, how to evaluate employee performance, and so on.
So, even if you have one or two employees, it's useful to "plan like the big boys" regarding human resource and personnel issues. The methods you use may be simpler but you still need to do it, so you are prepared.