Calculating Median Value within a Group By
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; -- replace WITH (nolock)

SELECT [Time]
       ,AVG(NULLIF(Duration,0)) AS [Mean Response Time]
       ,AVG(CASE WHEN Rn in (Round(CN/2.0,0),Round((CN+1)/2.0,0))
                 THEN Duration END) [Median]
FROM
(
  SELECT [Time], Duration,
         Rn = Row_Number() over (partition by CASE WHEN NullIf(Duration,0) is null then 1 else 2 end,
                                         Time
                                 order by Duration),
         Cn = Count(1) over (partition by CASE WHEN NullIf(Duration,0) is null then 1 else 2 end,
                                         Time)
  FROM (
    SELECT dateadd(minute, 10 + (datediff(minute, 0, [Started]) / 10) * 10, 0) AS [Time],
           Duration
    FROM [Application].[Exchange]
    WHERE [Started] &gt;= '20121024' AND [Started] &lt; '20121025'
  ) X
) Y
GROUP BY [Time]
ORDER BY [Time];