3 ms·

What is the formula used for confidence interval ? "0.0%-106.9%" interval for low numbers is somewhat disturbing. What approximation cause this problem ?

• madez 11y ago
  You can have an annual failure rate of over 100%. It'd mean that the expectation of the duration until all your drives failed is less than one year.
• wtallis 11y ago
  The 106.9% isn't a problem of mathematics; it just means an average of more than one failure per drive per year, or alternatively an average lifespan of less than one year. It comes from the fact that they've only had those models in service for a very short time, and so they can only constrain the lifetime through the same reasoning used to estimate a MTBF of multiple years with less than a year of testing. With zero failures so far, it's obvious why the bottom of the confidence interval is at 0%.
• dragontamer 11y ago
  They only have 50 Toshiba drives, which have only been used for a couple of months. With 0% failures over the 50 drives, you've got a statistical nightmare. You really have _no_ idea how reliable or unreliable the drives are. So the "confidence interval" is huge. The other drives have thousands and thousands of samples, and have been used for many years. They are confident in the measurement of the reliability. Toshiba has taken over the former Hitachi factories (along with HGST). So I'd expect Toshiba drives to be highly reliable. I hope that Backblaze can purchase more Toshiba drives so that they can gather some better statistics. For now, I'm confident in their 50 hard drives that haven't failed. That's a pretty good measurement, even if its not statistically reliable quite yet.