Deceptive Numbers: Sprinklers vs Smoke Alarms

Statistics can be very powerful pieces of information. It is a great way to take large areas of information and distill it into easily digestible pieces. But without a good understanding of math, it is easy to be deceived.

I saw an example of this in a recent issue of Building Products:

A March 2010 report from the National Fire Association found that over five years, the death rate per 1,000 reported home fires was 83% lower when sprinkler systems were present than when they weren’t.

Eighty-three percent seems like a huge number, right? Think of all the lives that sprinklers must save. We MUST put them into every single house in America. Think of the children!

Hold on before you start buying copper pipe. The 83% is a relative number; it is a percentage of the death rate, which itself is simply a percentage of all fires. You need to know how dangers house fires are before you can really evaluate the actual improvement sprinklers would cause.  Luckily, we can get our information from the very same article:

The chance of surviving a fire in a dwelling with working smoke alarms – required in new homes in some form since the late 1970s – is 99.45%, a number borrowed from the same NFPA report.

This statistic means that almost every single house fire with smoke alarms is survived. Suddenly, that 83% reduction is not as impressive as it sounds. Spending thousands of dollars to install sprinklers, then hundreds in maintenance over the years after, only changes the absolute survival by about half a percentage.

Compare the numbers concerning smoke alarms. Adding smoke alarms to a home reduces the death rate by 50% versus houses without it, which is worse than the 83% right?  The sprinkler number was reducing an already small number even smaller. The chance of survival survival in a house without smoke alarms? A mere 66%.

A smoke alarm costs around ten dollars. Installing hard-wired, interconnected alarms throughout the house costs a few hundred dollars. For that low price, smoke alarms take survival from two-thirds to a near certainty.

Does this mean residential sprinklers are useless? Of course not. But their valve is only an incremental increase over the already extremely high safety of smoke alarms, despite what a cursory glance at the numbers might make you think.

-That is all.

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