15.4. Understanding the sources and impact of missing data

We identify the amount, distribution, and pattern of missing data in order to evaluate (1) the potential mechanisms producing the missing data and (2) the impact of the missing data on our ability to answer substantive questions. In particular, we want to answer the following questions:

• What percentage of the data is missing?

• Is it concentrated in a few variables, or widely distributed?

• Does it appear to be random?

• Does the covariation of missing data with each other or with observed data suggest a possible mechanism that’s producing the missing values?