The holmium laser, which operates in a wavelength of 2.1 microns, is highly absorbed by water and has a shallow penetration depth of approximately 0.4 mm. As a pulsed mode laser, the Holmium reduces deeper thermal effects and charring which are more prevalent in Continues Wave lasers (e.g. Thulium, KTP).
In statistical theory, covariance is a measure of how much two random variables change together. In other words, covariance is a measure of the strength of the correlation between two random variables. Also, it can be considered as a generalization of the concept of variance of two random variables.
Covariates as Control Variables. But the other part of the original ANCOVA definition is that a covariate is a control variable. So sometimes people use the term Covariate to mean any control variable. Because really, you can covary out the effects of a categorical control variable just as easily. In our little math training example, you may be unable to pretest the participants. Maybe you can only get them for one session.