Writing different forms of text at different levels for responding satisfactorily to issues in the academic community is important. Thus, this paper examines the statistical differences between tertiary students' academic writing skills in different disciplines and at three different levels. We use a quantitative approach and descriptive design to extract and analyse data comprising students' essays from four departments of a public university in Ghana. We analyse the data using descriptive statistics and ANOVA in SPSS. The results show that there is a significant difference between students' academic writing skills across levels within a discipline. On the contrary, there is no significant difference between students' academic writing skills across departments. We successfully generated corpus data and analysed the same statistically which is not common in the English as a second language learning context, thereby contributing greatly to the academic environment. This paper also focused on the students' general vocabulary knowledge as a crucial factor in their academic writing skills.