In the context of your research, you could view “failed” and “no suitable probes” as finished, but here is a complete list of possible measurement statuses:

• Specified: the measurement has been defined and sent to our infrastructure to be relayed to the probes.
• Scheduled: The measurement has been relayed to the probes. If the start time is immediate, this status doesn’t last very long.
• Ongoing: The measurement is running on available probes.
• Stopped: The measurement was stopped either on schedule, by the user requesting an early stop.
• Forced to Stop: The measurement was killed prematurely due to a lack of available credits.
• No suitable probes: The measurement cannot currently be executed as defined due to a lack of available probes. This may be because you asked to use probes that don’t exist (for example, probes in an AS in which there are no probes) or because all of the probes you requested were too busy to take on new jobs. This latter scenario is very rare however.
• Failed: If a probe never actually runs a single measurement over the duration of the specified start/stop time (typically due to a lack of available probes), it will be marked as Failed once the stop time has been reached.

In other words, Specified, Scheduled, Ongoing, and No suitable probes are statuses potentially assigned to running measurements (or at least those who may return results at some point), while Stopped, Forced to Stop, and Failed are statuses assigned to measurements that will not be returning any more results.