# Missing Data: Part 1 What to Do? Carol B. Thompson Johns Hopkins Biostatistics Center SON Brown Bag 3/20/13

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1 Missing Data: Part 1 What to Do? Carol B. Thompson Johns Hopkins Biostatistics Center SON Brown Bag 3/20/13

2 Overview Missingness and impact on statistical analysis Missing data assumptions/mechanisms Conventional but not best options Preferred options Maximum Likelihood Multiple Imputation Inverse Probability Weighting 2

3 Statistical Analysis & Missingness Sampling process Population to which inference is to be made Inference process Sample used for inference (assume representative) Part of sample with missing data Is sample with missing data still representative enough to make appropriate inferences to population of interest????? 3

4 Completers vs Missing (Schafer) 4

5 Handling Missingness Impact How do cases with missing data compare to those without missing data? (missingness mechanism) What assumptions in the analysis can be made to either adjust for or account for effect of missing data? GOAL to make estimates or inferences with missing data that can apply to population as if complete information were available 5

6 Analysis Goals When Data Are Missing Minimize bias (representative of Population) Maximize use of available information Good estimates of uncertainty (std errors and p-values) Method is easy to use NOT that imputed values are close to real values 6

7 Missingness Mechanisms Process by which observations become missing Can t be known for sure; expressed only by observed data Mechanism types Missing Completely at Random (MCAR) Missing at Random (MAR) Missing Not at Random (MNAR) 7

8 MCAR Missing Y are unrelated to value of Y or any variable to be analyzed with Y Can use complete data records as simple random subsample from full data set Missing Y may be related to some other X: missing age related to failure to report income Example - Lab sample dropped MCAR example? Subject falls under bus (what if psych trial?) 8

9 MCAR cont d Violation: missing income related to age Compare age for missing Y and non-missing Y No difference does not indicate MCAR MCAR may occur because of research design Two evaluations, one is very expensive Everyone receives less expensive one Random sample receives very expensive one MCAR is strongest assumption Unbiased estimates and don t need to model missingness 9

10 MAR Weaker assumption than MCAR Missing Y are not related to value of Y, after controlling for other X in the analyses 2 subjects with same observed values have same statistical behavior on other variables, observed or not MAR - Ignorable 10

11 Example: MAR - Ignorable Missing income depends on mental status Within mental status category, missing income not related to value of income (i.e., when controlling for mental status). Not possible to test for MAR, because no info on missing Y 11

12 MAR Ignorable cont d Parameters governing missing data process are unrelated to parameters to be estimated Rare that above assumption is not satisfied No need to model missing data mechanism If assumption is not satisfied, ignorability methods work fine, but it might be better to model missing data mechanism 12

13 MAR Non-Ignorable Missing Not at Random (MNAR) Even accounting for all available information, the reason for missing observations still depends on the unseen data Example In clinical drug trials, subjects getting worse are more likely to drop out Must model missingness mechanism Requires some prior knowledge to do this Best to perform sensitivity analysis on likely models for missingness mechanism 13

14 Conventional But Not Best Options Listwise deletion Pairwise deletion Dummy variable adjustment Last observation carried forward (LOCF) Imputation Simple mean Regression mean 14

15 Listwise Deletion Use only complete cases Advantages No restriction on type of analysis performed No special methods required If MCAR, can be considered simple random subsample Unbiased estimates of standard errors and test statistics are appropriate Std errors may be larger because fewer cases 15

16 Listwise Deletion cont d Disadvantages If MAR, estimates may be biased. If multiple models, cases used per model may change inconsistent inferences. Robust for violations of MAR Assumes regression coefficients are same for all cases If relationships vary across subpopulations, may weight coefficients toward one group or another (separate regressions or interactions) 16

17 Pairwise Deletion AKA: Available case analysis Works for analyses depending only on summary statistics Means, std deviations, correlations Not good for regression, factor analysis 17

18 Pairwise Deletion cont d Ambiguities in implementation: If MCAR, estimates are approximately unbiased for large samples If MAR, but not observed at random, estimates and std errors may be seriously biased. Need sample size to calculate std errors, but N from which pair of variables??? Not a good alternative to Listwise Deletion 18

19 Dummy Variable Adjustment Create variable D based on whether X s value is missing or not (0,1) Adjust X: Xc = X if not missing Xc = c if missing (value of c doesn t matter) Regress Y on Xc, D, other X 19

20 Dummy Variable Adjustment cont d Advantages Uses all available data Choice of c only affects coefficient for D Coefficient for D = predicted Y if missing X, controlling for other X Coefficient for Xc = estimate of effect of X if not missing X 20

21 Dummy Variable Adjustment cont d Disadvantages Biased estimates even if MCAR Impact of extra category in variable for missing value Depends on distribution of missing responses across rest of categories Depends on how probability of being missing depends on category Dissimilar cases can be grouped together in missing category Will likely lead to severe bias 21

22 LOCF Usually used in longitudinal studies Assumes that future observations on a subject are the same as past observations Realistic assumption under treatment??? Means and covariance structure are seriously distorted 22

23 Imputation Substitute with reasonable guess; analyze as if no missing data or guesses Mean imputation Substitute with marginal mean (mean of all observations with data on variable) Biased estimates of variances and covariances AVOID Substituting mean for items in scale to get total Consider # missing items perhaps < 25%, and # cases with missing items 23

24 Imputation cont d Conditional or Regression Mean Imputation Regress missing X on all other X s to generate predicted values of X Use mean predicted value for each missing X Can give unbiased estimates of mean, associations and regression coefficients in more situations than simple mean imputation 24

25 Imputation cont d If missing is only on 1 X If MCAR, coefficients are approximately unbiased in large samples Can use with weighted least squares or generalized least squares These imputations underestimate standard errors and thus overestimate test statistics (more significance than warranted). 25

26 course_grade Imputation Example (Baraldi) Complete Data Set List-wise Deletion math_aptitude math_aptitude Mean 76.35; Std Dev Mean 81.8, Std Dev

28 Conventional but not best options make things worse Introduce substantial bias Make analysis more sensitive to departures from MCAR Yield std error estimates that are incorrect (usually too low) potential for more significance than warranted If # missing is small, may not affect much, but how small or what makes up missing may not be defensible. 28

29 Preferred Options Maximum Likelihood Estimation (MLE) Multiple Imputation (MI) Inverse Probability Weighting (IPW) Weight observations inversely according to the estimated probability of not being missing 29

30 Preferred Options cont d They make distributional assumptions about The unseen data Form of MV mechanism Based on well-defined statistical models Don t replace MV directly Available info is combined with assumptions; doesn t depend on observed data alone Analyses, inferences, conclusions are valid under these assumptions 30

31 MLE Identifies parameter values that have the highest probability of producing the observed data. Under a fairly wide range of conditions: Approximately unbiased Asymptotically normal large samples Can be used to construct CIs and p-values 31

32 MLE General Missing Patterns Expectation-maximization (EM) Basically reduces to regression imputation of missing values (E) Computes means and covariance matrix with imputed values (M) Cycles through E and M until estimates converge Uses all variables as predictors Advantages Easy to use; lots of commercial software Disadvantage Linear modeling std errors + test statistics are incorrect 3/20/2013 Biased estimates SON_BB_MissingData_part1_

33 MLE General Missing Patterns cont d Direct ML Uses structural equation modeling (e.g., AMOS, MPlus) Advantages Estimates optimal for large samples Estimates with correct std errors Disadvantage Requires specialized software Some distribution assumptions may be unrealistic 33

34 Multiple Imputation (MI) MI has same optimal properties as ML, but removes some of ML limitations When used correctly, produces estimates: Approximately unbiased, better as sample size increases Asymptotically normal when data are MAR Can construct CIs and p-values 34

35 Advantages MI cont d Can be used with virtually any kind of data, any kind of model, and with unmodified conventional software Disadvantages Can be cumbersome to implement, Is easy to do wrong, Produces different estimates every time it is used (hopefully small differences) 35

36 Single Random Imputation Replaces missing value with random draw from a distribution based on observed data (hot-deck) Does not produce a unique set of numbers (random variation in process) Without random component, produces underestimates of variances for variables with missing data. 36

37 MI Process Repeat the random imputation process more than once (5 times is generally enough) Each imputation process represents random sample from distribution of plausible values for missing values Important for imputation processes to be independent large number of iterations between each saved data set Analyze data set from each imputation process as if no missing data 37

38 MI Process Pooling Estimates Calculate mean of estimates Calculate mean of squared std errors Calculate variance of estimates Calculate square root of mean of variances plus variance of estimates Can be used with any parameter 38

39 MI Example (Howell- Part 2) 39

40 Additional Rules of Thumb (Allison) Dependent Variable (DV) should always be included in imputation regression analysis Impute missing values on DV if: There are auxiliary variables strongly correlated with DV. Don t impute DV if: No missing predictor data or auxiliary variables No auxiliary variables and missing predictor data 40

41 References ing.html - Howell DC. Treatment of Missing Data _ Part I ing-part-two.html - Howell DC. Treatment of Missing Data _ Part II Allison, Paul D. Missing Data Sage Publications (Series: Quantitative Applications in the Social Sciences), Thousand Oaks, CA. Baraldi AN, Enders CK. An introduction to modern missing data analyses Journal of School Psychology 48: Schafer J. Missing Data in Longitudinal Studies: A Review (2005) 41

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