Which type of value is stored for an attribute when the value or quality changes, according to Historian storage methods?

The correct answer is analog, as this type of value represents continuous data that can change over time. In the context of a Historian server, analog values are typically used to store measurements that can vary in a smooth way, such as temperature, pressure, or flow rates.

When the quality or value of an analog attribute changes, the Historian captures this data to maintain an accurate representation of the process. This ensures that users can analyze trends and fluctuations over time, which is essential for process control and monitoring.

While discrete values refer to distinct and separate states (often binary), and calculated values stem from formulas applied to the existing data, they do not directly convey the continuous variability that is characteristic of analog descriptions. Historical data generally encompasses past values but does not specifically imply the nature of change, making analog the appropriate classification for this scenario.