Compute Quantiles or Bucketing/Binning

SELECT
  trafficSource.medium,
  SUM(totals.visits) AS sessions,
  -- 5 will give the min, 25%, 50%, 75%, max with 20% error
  -- the more buckets, the better the approximation (error = 1/number of buckets)
  -- at the cost of more computation
  -- QUANTILES returns all of the buckets, use NTH to extract the bucket you want
  NTH(2, QUANTILES(totals.timeOnSite, 5)) AS firstQuartile,
  NTH(3, QUANTILES(totals.timeOnSite, 5)) AS mean,
  NTH(3, QUANTILES(totals.timeOnSite, 5)) AS thirdQuartile
FROM (
  SELECT
    trafficSource.medium,
    totals.visits,
    -- Sessions with a single page view will have no time on site reported
    IF(totals.timeOnSite IS NULL, 0, totals.timeOnSite) AS totals.timeOnSite
  FROM
    [google.com:analytics-bigquery:LondonCycleHelmet.ga_sessions_20130910])
GROUP BY
  trafficSource.medium