Measurement error (i.e. the complement of reliability) results in a downward bias of observed effect sizes. This attenuation can be emulated by this function.
The value of Cohen's d (that would be obtained with perfect measurements)
The reliability of the measurements of the continuous variable
The attenuated value of Cohen's d
Bobko, P., Roth, P. L., & Bobko, C. (2001). Correcting the Effect Size of d for Range Restriction and Unreliability. Organizational Research Methods, 4(1), 46–61. doi: 10.1177/109442810141003
attenuate.d(.5, .8); #>  0.4472136