Perhaps you should try some spelling variations:
observational economy » observational astronomy, observational cohort, observational cinema
observational model » observation model, observation models, observations models
observation concept » observation concepts, conservation concept, observation content

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((( observational model ) OR ( observation concept ))) OR ( observational economy )).
• Adding a wildcard symbol may retrieve word variants: (((( observational model ) or ( observation concept ))) or ( observational economy ))*.