Given the task definition and input, reply with output. Given a sentence in Igbo language, translate the sentence to English language keeping the meaning of the original sentence intact

Igbo sentence: A puru itinye covariates na-eji gwugw eme ihe na nhazi na nyocha nke nyocha; isi nke 4 nke Gerber and Green (2012) enye nkwalite di mma ma biaruo nso, na Casella (2008) -enyekwu nhazi miri emi.
Pre-treatment covariates can be included in the design and analysis stage of experiments; chapter 4 of Gerber and Green (2012) provides a good introduction to both approaches, and Casella (2008) provides a more in-depth treatment.