tetrachoric correlation coefficient: a correlation coefficient computed for two normally distributed variables that are both expressed as a dichotomy
correlation coefficient: a statistic representing how closely two variables co-vary; it can vary from -1 (perfect negative correlation) through 0 (no correlation) to +1 (perfect positive correlation)
whipple's penstemon: wine and lavender to purple and black flowers in several clusters on the upper half of leafy stems; Montana south through the Rocky Mountains to Arizona and New Mexico