Alternate quartile calc algorithm

With SourceData AS
(
    SELECT GroupID, CONVERT( decimal(20, 10), Measure) Measure
    FROM dbo.TEST_MEDIAN
    UNPIVOT(Measure FOR GroupID IN (Computer, Laptop, Mouse)) AS normalized
)
SELECT GroupID,
    (Max(Q1LoVal) + Max(Q1HiVal)) / 2 Quartile1,
    (Max(Q2LoVal) + Max(Q2HiVal)) / 2 Median,
    (Max(Q3LoVal) + Max(Q3HiVal)) / 2 Quartile3,
    Min(Measure) Minimum, Max(Measure) Maximum
FROM (
    SELECT detail.GroupID,
        CASE WHEN RowNum = Q1Lo THEN Measure ELSE NULL END Q1LoVal,
        CASE WHEN RowNum = Q1Hi THEN Measure ELSE NULL END Q1HiVal,
        CASE WHEN RowNum = Q2Lo THEN Measure ELSE NULL END Q2LoVal,
        CASE WHEN RowNum = Q2Hi THEN Measure ELSE NULL END Q2HiVal,
        CASE WHEN RowNum = Q3Lo THEN Measure ELSE NULL END Q3LoVal,
        CASE WHEN RowNum = Q3Hi THEN Measure ELSE NULL END Q3HiVal,
        Measure
    FROM
        (SELECT *,  ROW_NUMBER() OVER (PARTITION BY GroupID ORDER BY Measure) RowNum
        FROM SourceData) AS detail
    INNER JOIN (
        SELECT GroupID,
            CEILING(FLOOR(COUNT(*) / 2.0) / 2) Q1Lo,
            CEILING(CEILING(COUNT(*) / 2.0) / 2) Q1Hi,
            (COUNT(*) + 1) / 2 AS Q2Lo,
            (COUNT(*) + 2) / 2 AS Q2Hi,
            (COUNT(*) + 1) - CEILING(CEILING(COUNT(*) / 2.0) / 2) Q3Lo,
            (COUNT(*) + 1) - CEILING(FLOOR(COUNT(*) / 2.0) / 2) Q3Hi,
            COUNT(*) Ct
        FROM SourceData
        GROUP BY GroupID
        HAVING COUNT(*) > 1
    ) AS summary ON detail.GroupID  = summary.GroupID
) AS combined
GROUP BY GroupID;