Missing data occur when no data value is stored for the variable in the current observation. Missing data are a common occurrence and can have a significant effect on the conclusions that can be drawn from the data. There are different patterns of missing data: completely at random, at random or not at random.
The book Statistical analysis with missing data (Little RJA, Rubin DB. 2nd ed.,Wiley 2002) describes many aspects of the handling of missing data. The section ‘Handling of missing values’ in Rothman’s Modern Epidemiology, 3rd ed. (K. Rothman, S. Greenland, T. Lash. Lippincott Williams & Wilkins, 2008) is a summary of the state of the art, focused on practical issues for epidemiologists. Ways of dealing with such data include complete subject analysis (subjects with missing values are deleted from the analyses) and imputation methods (missing data are predicted based on the observed values and the pattern of missingness). A method commonly used in epidemiology is to create a category of the variable, or an indicator, for the missing values. This practice can be invalid even if the data are missing completely at random and should be avoided (see Indicator and Stratification Methods for Missing Explanatory Variables in Multiple Linear Regression. J Am Stat Assoc 1996;91(433):222-30).
A concise review of methods to handle missing data is provided in the section ‘Missing data’ of the Encyclopedia of Epidemiologic Methods (Gail MH, Benichou J, Editors. Wiley 2000). Identifying the pattern of missing data is important as some methods for handling missing data assume a defined pattern of missingness. Biased results may be obtained if it is incorrectly assumed that data are missing at random. In general, it is desirable to show that conclusions drawn from the data are not sensitive to the particular strategy used to handle missing values. To investigate this, it may be helpful to repeat the analysis with a variety of approaches.
Other useful references on handling of missing data include the books Multiple Imputation for Nonresponse in Surveys (Rubin DB, Wiley, 2004) and Analysis of Incomplete Multivariate Data (Schafer JL, Chapman & Hall/CRC, 1997), and the articles Using the outcome for imputation of missing predictor values was preferred (J Clin Epi 2006;59(10):1092-101), Recovery of information from multiple imputation: a simulation study (Emerg Themes Epidemiol 2012;9(1):3) and Evaluation of two-fold fully conditional specification multiple imputation for longitudinal electronic health record data (Stat Med. 2014;33(21):3725-37).