OP06: Average length of observation, in days

Count average number of observation days. 

Input:
<None>

Sample query run:
The following is a sample run of the query:
SELECT avg( observation_period_end_date 
          - observation_period_start_date ) AS num_days
  FROM observation_period
Output:
Output field list:
 Field  Description
num_days Average length of observation, in days

Sample output record:
 Field  Value
num_days 731
Comments