What Is MozCast?

MozCast is an experiment in tracking the "weather" patterns of the Google algorithm. While big updates like Panda and Penguin get a lot of press, Google revealed that they made 516 changes in 2010, and all evidence suggests that pace has continued. What we notice is barely the tip of the iceberg. MozCast was designed to help you keep track of the day-to-day changes in the Google algorithm.

How Does It work?

It all starts with this: every 24 hours, we track a hand-picked set of 1,000 keywords and grab the top 10 Google organic results. Keywords were deliberately chosen to avoid obvious local intent, are distributed evenly across 5 "bins" by query volume, and are tracked at roughly the same time every day from the same location. In other words, we do our best to keep the data set tightly controlled.

Each day, we take the current top 10 and compare it to the previous day's top 10 (for any given keyword), and calculate a rate of change or "delta". Specifically, we use two metrics – called Delta 100 and Delta10 – which measure the change across the entire set of top 10 results. Then, we crunch Delta10 across all 1,000 keywords and multiply the resulting average by a fixed multiplier (currently, 28.0) to produce something that looks like a temperature (in °F). An average day is about 70°F.

So, What's a Delta10?

Ok, here's the more complicated version of the story. We tried a lot of metrics for comparing two sets of top 10 results, and we ultimately landed on two that seemed to work pretty well but were simple enough to explain. The first is "Delta100" – for any two days of top 10 rankings, it gives us a number from 0-100, calculated by the following rules:

  • +0 if the ranking stays the same
  • +|#| if the ranking moves in the top 10
  • +10 if the ranking drops out of the top 10

If a ranking moves, then "#" is the number of spots it moves – in other words, if a URL drops from the #2 spot to the #5 spot, then +3 gets added to Delta100 for that keyword. Across all 1,000 keywords, Delta100 looks like a power curve, so to normalize the data (and make statistics like the mean and standard deviation easy to calculate), we take the square root of Delta100 to create Delta10 (called that because it theoretically ranges from 0-10). All temperatures and long-term reports on MozCast are directly based off of Delta10.

What Does It Mean?

Put simply, the hotter the temperature, the more change we saw in Google rankings in the previous 24 hours. Each temperature also gets converted into one of five severity levels:

weather severity levels

The temperature and severity are completely connected at this point, so a big algorithm change means you'll see hot and stormy weather. Again, the weather part is just for fun. Think of the severity as the at-a-glance view and the temperature as the refined view (a direct representation of our internal Delta10 metric). If it's a stormy day, then something big happened.

Stay Tuned...

Data updates automatically every morning (~7:30am Pacific Time), and the 5-day history is available from the left sidebar. Check out the Google Algorithm Change History on moz.com for a list of how recent algorithm updates have scored on our scale. Follow us on Twitter @mozcast for your daily weather report.