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SELECT element, AVG(num_obs) AS avg_number_obs
FROM (
    SELECT s.continent_name as continent, e.name as element, COUNT(*) AS num_obs
    FROM factobservation AS o
    JOIN dimelement AS e ON o.element_id = e.id
    JOIN dimstation AS s ON o.station_id = s.id 
    GROUP BY continent, element
) AS obs_counts
GROUP BY element;