I have a data file containing educational test results for students in year levels 3, 5, 7, and 9. Each year level also has a value for "assessed year", based on the calendar year in which the student sat the test (values for calendar year vary by id). However I have many missing values for "assessed year". For each id, each value of assessed year should be 2 years apart. This is what the data should look like: ID YearLevel AssessedYear 1 3 2007 1 5 2009 1 7 2011 1 9 2013 2 3 2009 2 5 2011 2 7 2013 2 9 2015 3 3 2011 3 5 2013 3 7 2015 3 9 2017 4 3 2003 4 5 2005 4 7 2007 4 9 2009 5 3 2003 5 5 2005 5 7 2007 5 9 2009 But this is what I have: ID YearLevel AssessedYear 1 3 . 1 5 2009 1 7 . 1 9 2013 2 3 2009 2 5 2011 2 7 2013 2 9 . 3 3 2011 3 5 . 3 7 . 3 9 . 4 3 . 4 5 2005 4 7 2007 4 9 2009 5 3 . 5 5 . 5 7 . 5 9 2009 I want to fill in the missing cells by calculating +/- 2 years from the non-missing values of assessed year. Any suggestions will be greatly appreciated, this has been keeping me occupied for some time now! Thanks
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