Copywriting tricks: What does 99.9% actually mean?

I read today in New Media Age that "unreliable and slow sites resulted in etailers losing £84m in the final three months of 2005." My web hosting company boasts that it has 99.9% server uptime. This means that they expect at least eight hours of downtime a year. Some clever copywriter made a deal with the marketing analytics devils there. That 0.01% has a real money cost.

In Moondust, Andrew Smith's fascinating book about the moon missions, it says that the Saturn V rocket had six million parts, "meaning that, even with NASA's astounding 99.9 per cent reliability target, roughly 6,000 things could be expected to go wrong on a good flight."

There are two points here. First, impressive-sounding numbers are often not so impressive on closer examination. Nobody says "our servers fail for at least eight hours every year" because 99.9% uptime sounds better but they mean the same thing.

Second, this ambiguity is why genuine guarantees are so valuable in business writing. For example Symantec.cloud promises "100% protection from both known and unknown malware."

When it comes to e-commerce, space rockets and virus protection, there's a big difference between 100% and 99.9%.