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From the Speedtest.net site: Download Speed Your computer downloads small binary files from the web server to the client, and we measure that download to estimate the connection speed. Based off this result, we choose how much data to download for the real test. Our goal is to pick the right amount of data that you can download in 10 seconds, ensuring we get enough for an accurate result, but not take too long. We prevent caches from throwing off results by appending random strings to each download. Once we start downloading, we use up to four HTTP threads to saturate your connection and get an accurate measurement. Throughput samples are received at up to 30 times per second. These samples are then aggregated into 20 slices (each being 5% of the samples). The fastest 10% and slowest 30% of the slices are then discarded. The remaining slices are averaged together to determine the final result. Why do we discard certain results? We want to ensure we're giving you the most accurate assessment of your connection's maximum sustained throughput. Here's how we do that: Outlying 10%: Since we're measuring data transported over HTTP (via Flash), speed can be affected by a few things, such as potential protocol overhead; buffering due to the many layers between our application and the raw data transfer; or throughput bursting due primarily to CPU usage. To account for these variables, we initially drop the top 10% and bottom 10% of our slices as outliers. Test Ramp-Up Period: We keep the default test length short to improve user experience, but in doing so, the ramp-up period can take up a significant portion of the beginning of the test. In consideration of that, we also drop another 20% of the bottom result slices. ----- Speedtest is being much more direct in explaining how the results are calculated - as compared to only a few months ago. Good for them. My observations from comparing Speedtest.net and testmy.net are first, the results cannot be rationally compared. Testmy.net results are calculated using the complete test while speedtest.net results are form a 60% slice of the total test. I also see a difference in how Speedtest is (seemingly) implemented by various ISPs. Speedtest.net tests are observed to take from 3 to 8 seconds depending on which ISP hosted test I use. As has been already noted, there is typically a surge in speed during the first few seconds of download. A Speedtest result that is obtained after only a few seconds provides me with a result which averages 150% of the throughput I pay for. That speed in not sustained, however. Any test, testmy.net or Speedtest.net that takes 7 seconds or more shows the result of speed settling back to or below my paid for throughput. My opinion is that various uses of the Internet impact how we view speed. Using testmy.net I can see the chart illustrating the ramp-up of connection - the subsequent speed burst - the typically sustained throughput - and finally, the tailing off as the download is completed. The early burst in speed is certainly appreciated when simply browsing or checking email. The snappiness provides a warm and fuzzy feeling. When I am streaming video for an extended period then the sustained throughput at the speed I pay for is important. It is no wonder that most ISPs use Speedtest.net as their official throughput test. It usually makes them look good.