Analysing Data with 3 Variables (2 numerical and 1 categorical)


I'm currently completing a research report in the field of education and I'm having difficulty working out how to analyse data with three variables. There is a known correlation between two numerical variables of schools (ICSEA which is an indicator of SES and VCE median study score). The correlation is that as ICSEA increases, VCE median study score also increases. The third variable is about what type of device policy schools have and is categorical (school device, byo - specified device, or byo - any device). I'm trying to see if the device policy of a school affects student learning outcomes across quarters but I'm unable to make a fair comparison as the average ICSEA of each category is different. Is there a calculation that I can make in excel to account for this difference?

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Thank you in advance for any suggestions or assistance!