What is Apdex Score?

The Apdex Score (Application Performance Index) is a numerical measure of user satisfaction with the performance of web applications and services. It was developed by the Apdex Alliance in 2004 to provide a simple, consistent way to evaluate responsiveness from the user's perspective. The score ranges from 0 to 1, where 1.0 means all users are satisfied and 0 means no users are satisfied.

To calculate the Apdex Score, an organization first defines a response time target (T) in seconds, typically around 0.5 to 2 seconds depending on the application. All requests that complete in less than or equal to T are considered satisfied. A second threshold is set at 4T; requests that finish between T and 4T are classified as tolerating. Any request exceeding 4T or resulting in an error is deemed frustrated. The score is then computed as (number of satisfied requests plus half the number of tolerating requests) divided by the total number of requests.

In the context of content delivery networks and web performance monitoring, the Apdex Score offers a more nuanced view than simple average latency. It captures the proportion of users who actually have a good experience, rather than being skewed by outliers. Many performance monitoring tools and CDN analytics dashboards report Apdex Scores alongside other metrics like Time to First Byte and Largest Contentful Paint. The score is commonly used in service-level agreements to define acceptable performance thresholds, with a score of 0.94 or higher typically considered excellent.