Measurement error (i.e. the complement of reliability) results in a downward bias of observed effect sizes. This attenuation can be reversed by disattenuation.
The (attenuated) value of Cohen's d (i.e. the value as observed in the sample, and therefore attenuated (decreased) by measurement error in the continuous variable).
The reliability of the measurements of the continuous variable
The disattenuated 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
disattenuate.d(.5, .8); #>  0.559017