sum of squares or square of total?

Question

According to https://newrelic.com/docs/plugin-dev/metric-data-for-the-plugin-api plugins should also report the sum of squares for the specified period. Taking a look at the JSON example, I'd say this is more a square of the total value. E.g.:

{
  "name": "Primary MySQL Database",
  "guid": "com.your_company_name.plugin_name",
  "duration" : 60,
  "metrics" : {
    "Component/ProductionDatabase[Queries/Second]": 100,
    "Component/AnalyticsDatabase[Queries/Second]": {
      "min" : 2,
      "max" : 10,
      "total": 12,
      "count" : 2,
      "sum_of_squares" : 144
    }
  }
}

Obviously there were two values, 2 and 10. According to the sample the sum_of_squares is (10+2)^2=144 which I would define as "square of total". For me however, the term "sum of squares" would be 2^2+10^2=104.

So when taking about multi-value metrics - which one is the correct number?

Answer 1

You're absolutely right that the example values as documented are incorrect. I'll notify the relevant parties to update the documentation.

In the common case where only a single metric value is being reported, the count will be (1), in which case squaring the "total" value yields the correct result.