What are important points to consider when being asked to analyse the usefulness of a primary source?

In order to answer a primary source there are 4 main points of focus in order to ascertain how useful the source is:Who - did the author (if it were a written artefact) have a particular reason for reacting to an event in the way they did. If we consider a soviet historian writing in the time of Stalin this would provide an explanation as to why their writing expresses certain political views, and can be used to weigh up the usefulness of the sourceWhat - what form does the source take, newspaper article, diary entry, document of wages or statistics, does this effect the way we view the source or limit its usefulnessWhere - where was the source written, was the person writing this source in a position to make a well founded claim on the actions they are recordingWhen - when was the source written, had public opinion shifted since the event and the writing of the source, was it written long after the event.