The answer is easy. Both!
A recent article in Industry Week on The MRO Dilemma asked if you should focus on safety stock or service levels. The answer is both.
The article, which notes that waste is generated every time a piece of equipment breaks down or runs at less than optimal speed because of needed repairs, and that these repairs are delayed if there is not enough spares on hand, notes that more MRO inventory translates to higher inventory carrying costs but also likely higher service levels while less inventory will reduce the carrying costs [while putting] service levels in jeopardy. This is obvious.
It is also obvious that trying to maintain 100% service levels is likely not an option for most companies because that would mean you just about built a duplicate plant in your store room.
But what might not be so obvious is that the 95% service level recommended as a good target is not good advice at all. The target service level does not, as the article indicates, depend on what your company can afford, but depends on what is optimal for your company. And it is often production line / product specific. A production line producing your most profitable product line should never be down, and if that dictates a costly 98% service level, so be it. However, the turn around time on replacing a printer in the admin offices is not nearly as critical and you can accept a service level of 90%, or less, from your internal IT support, especially if they have outsourced the function to a vendor and a higher service level would increase costs 20%.
Just like you optimize your buy, you optimize your service levels. If downtime on a production line costs you $1,000,000 per hour, you spend $100,000 to make sure you have spares for every moving part that can break. If downtime on a secondary machine that is only required for custom orders, which account for less than 10% of profits, only costs you $10,000 an hour, and stocking the same level of spares would cost you $50,000, you opt for a lower service level. It’s all about optimization.
And, there are companies like Servigistics and MCA Solutions, just to name a couple, that can help you optimize this trade-off so that you’re not improving inventory carrying costs at the expense of service levels and vice versa. With optimization, you can have both … at the right levels that are the most profitable for your organization. Be smart.