Add error rate definition

specs/agents/tracing-spans.md

@@ -17,8 +17,12 @@ the error rate of service B is 100% from service A's perspective.
 However, as service B doesn't receive any requests, the error rate is 0% from service B's perspective.
 The `span.outcome` also allows reasoning about error rates of external services.
 
+#### Outcome API
+
 Agents should expose an API to manually override the outcome.
 This value must always take precedence over the automatically determined value.
+The documentation should clarify that spans with `unknown` outcomes are ignored in the error rate calculation.
+
 #### Span stack traces
 
 Spans may have an associated stack trace, in order to locate the associated source code that caused the span to occur. If there are many spans being collected this can cause a significant amount of overhead in the application, due to the capture, rendering, and transmission of potentially large stack traces. It is possible to limit the recording of span stack traces to only spans that are slower than a specified duration, using the config variable `ELASTIC_APM_SPAN_FRAMES_MIN_DURATION`.

specs/agents/tracing-transactions.md

@@ -23,5 +23,25 @@ If an agent doesn't report the `outcome` (or reports `null`), the APM Server set
 
 What counts as a failed or successful request depends on the protocol and does not depend on whether there are error documents associated with a transaction.
 
+##### Error rate
+
+The error rate of a transaction group is based on the `outcome` of its transactions.
+
+    error_rate = failure / (failure + success)
+
+Note that when calculating the error rate,
+transactions with an `unknown` or non-existent outcome are not considered.
+
+The calculation just looks at the subset of transactions where the result is known and extrapolates the error rate for the total population.
+This avoids that `unknown` or non-existant outcomes reduce the error rate,
+which would happen when looking at a mix of old and new agents,
+or when looking at RUM data (as page load transactions have an `unknown` outcome).
+
+Also note that this only reflects the error rate as perceived from the application itself.
+The error rate perceived from its clients is greater or equal to that.
+
+##### Outcome API
+
 Agents should expose an API to manually override the outcome.
 This value must always take precedence over the automatically determined value.
+The documentation should clarify that transactions with `unknown` outcomes are ignored in the error rate calculation.