SELECT
*
FROM (
SELECT
     groupId
    ,agentId
    ,CAST ( RIGHT(impPackageCode, 4) AS INT ) as [Year]
FROM
    att_schema.att_nmfData
WHERE agentId IS NOT NULL
    AND ISNUMERIC(RIGHT(impPackageCode, 4)) = 1
    AND CONVERT(INT, RIGHT(impPackageCode, 4)) IN ( 2011, 2010 )
) x WHERE [Year] = '2011'

Msg 245, Level 16, State 1, Line 1
Conversion failed when converting the varchar value 'RBI' to data type int.