Now, explore the data by identifying categories and missing values. Drag various Dimensions to the Rows and Columns shelves to view categories, record counts, and missing values (Null). Compare some dimensions against each other to understand how they interact. Are both Null at the same time, or do they appear unrelated?

Tableau allows you to interact this way quickly, so take advantage of it. An example is shown in the following screenshot comparing the Measurement Flag fields for Qgag and Qpcp against each other. You can see that they are frequently Null at the same time: