Who has the largest mobile network? In the recent past, red or blue maps were flashed across the television screen, and we were left having to flip a coin, because it was impossible to tell which provider had better coverage. Missing completely was any mention of service level (voice call, or Skype video call, or streaming Netflix HD, as examples) or probability of getting that service level in a particular area.
We all want to know where we will be able to use our mobile devices -- Yosemite? Mammoth? Anza Borrego? -- and we think there's a map out there that will provide the answer. But, most maps are garbage. A good map will state that the coverage shown on the map is based on the probability of getting a certain level of service at a location.
A generic map may show nearly all of California as having mobile service, but that coverage might be based solely on voice calling, not broadband. If you want to stream YouTube, the coverage will likely shrink. Moreover, if you want to stream YouTube reliably, or use real-time streaming services without interruption, the coverage may shrink even more. Think: higher capacity = less coverage, and higher probability = less coverage.
In our study of mobile broadband coverage for California, we now incorporate probability into our maps. In the past, we used to display mobile "served" coverage estimates (i.e. greater than or equal to 6 megabits per second downstream AND 1.5 megabits per second upstream, or "6/1.5") based on average speeds measured in the field. That resulted in 98% of California households receiving 6/1.5 or faster. But, as I have written in earlier posts, "average" means that half of the time you will get better than 6/1.5, and half the time, worse.
Here is our served 6/1.5 household estimate from June 2013 using average speeds:
This is why we are now adjusting the average speed with the standard deviation. The standard deviation is a measurement of variability. The higher the standard deviation, the more variability. Reducing measured speeds by the mean (average) minus two standard deviations yields a much higher likelihood (98% in a normal curve) of getting 6/1.5 or faster at a particular location, but, that adjustment shrinks the coverage map. This higher threshold lowers the estimated household coverage to 16% of California.
Here is our served 6/1.5 household estimate from December 2014 using mean minus two standard deviations:
Remember, coverage maps are meaningless without specifying service levels and incorporating probability.