A detailed quality control (QC) should be an essential part of every statistical data analysis, since the quality of the data is crucial for the validity and generalizability of statistical results. The goal of quality control is not only to assess data quality, but also to verify the assumptions made or required for further data analysis. In this worksheet, we address the quality control of raw data. In the case of mchaptericroarray data, a second quality control after data preprocessing is necessary.