The Q-statistic or q-statistic is a test statistic:

• The Box-Pierce test outputs a Q-statistic (uppercase) which follows the chi-squared distribution
  • The Ljung-Box test is a modified version of the Box-Pierce test which provides better small sample properties
• The Tukey-Kramer test outputs a q-statistic (lowercase), also called the studentized range statistic, which follows the studentized range distribution