# Handling missing data in Stata a whirlwind tour

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1 Handling missing data in Stata a whirlwind tour 2012 Italian Stata Users Group Meeting Jonathan Bartlett 20th September /55

2 Outline The problem of missing data and a principled approach Missing data assumptions Complete case analysis Multiple imputation Inverse probability weighting Conclusions 2/55

3 Outline The problem of missing data and a principled approach Missing data assumptions Complete case analysis Multiple imputation Inverse probability weighting Conclusions 3/55

4 The problem of missing data Missing data is a pervasive problem in epidemiological, clinical, social, and economic studies. Missing data always cause some loss of information which cannot be recovered. But statistical methods can often help us make best use of the data which has been observed. More seriously, missing data can introduce bias into our estimates. 4/55

5 Untestable assumptions Whether missing data cause bias depends on how missingness is associated with our variables. Crucially, with missing data we cannot empirically verify the required assumptions. e.g. consider the following distribution of smoking status (for males in THIN from [1]): Smoking status n (% of sample) (% of those observed) Non 82,479 (36) (48) Ex 30,294 (13) (18) Current 57,599 (25) (34) Missing 56,661 (25) n/a Are the %s in the last column unbiased estimates? 5/55

6 A principled approach to missing data We cannot be sure that the required assumptions are true given the observed data. Data analysis and contextual knowledge should be used to decide what assumption(s) are plausible about missingness. We can then choose a statistical method which is valid under this/these assumption(s). 6/55

7 Outline The problem of missing data and a principled approach Missing data assumptions Complete case analysis Multiple imputation Inverse probability weighting Conclusions 7/55

8 Rubin s classification Rubin developed a classification for missing data mechanisms [2]. We introduce the three types in a very simple setting. We assume we have one fully observed variable X (age), and one partially observed variable Y (blood pressure (BP)). We will let R indicate whether Y is observed (R =1)oris missing (R =0). 8/55

9 Missing completely at random The missing values in BP (Y ) are said to be missing completely at random (MCAR) if missingness is independent of BP (Y )andage(x ). i.e. those subjects with missing BP do not differ systematically (in terms of BP or age) to those with BP observed. In terms of the missingness indicator R, MCAR means P(R =1 X, Y )=P(R =1) e.g. 1 in 10 printed questionnaires were mistakenly printed with a page missing. 9/55

10 Example - blood pressure (simulated data) We assume age has been categorised into and n = 200, but only 99 subjects have BP observed: Age n Mean (SD) BP (10.3) (11.7) 10 / 55

11 Checking MCAR With the observed data, we could investigate whether age X is associated with missingness of blood presure (R). If it is, we can conclude the data are not MCAR. If it is not, we cannot necessarily conclude the data are MCAR. It is possible (though arguably unlikely in this case) that BP is associated with missingness in BP, even if age is not. 11 / 55

12 Example - blood pressure (simulated data) We compare the distribution of age in those with BP observed and those with BP missing:. tab age r, chi2 row Key frequency row percentage r age 0 1 Total Total Pearson chi2(1) = Pr = p < from chi2 test, shows we have strong evidence that missingness is associated with age. 12 / 55

13 Missing at random BP (Y ) is missing at random (MAR) given age (X )if missingness is independent of BP (Y )givenage(x ). This means that amongst subjects of the same age, missingness in BP is independent of BP. In terms of the missingness indicator R, MAR means P(R =1 X, Y )=P(R =1 X ) 13 / 55

14 Checking MAR We cannot check whethe MAR holds based on the observed data. To do this we would need to check whether, within categories of age, those with missing BP had higher/lower BP than those with it observed. 14 / 55

15 BP MAR given age SBP (mmhg) /100 observed Observed Missing 27/100 observed Age (years) / 55

16 A different representation of MAR We have defined MCAR and MAR in terms of how P(R =1 Y, X )dependsonage(x )andbp(y ). From the plot, we see that MAR can also be viewed in terms of the conditional distribution of BP (Y )givenage(x ). MAR implies that f (Y X, R =0)=f (Y X, R =1)=f (Y X ) That is, the distribution of BP (Y ), given age (X ), is the same whether or not BP (Y ) is observed. This key consequence of MAR is directly exploited by multiple imputation. 16 / 55

17 Missing not at random If data are neither MCAR nor MAR, they are missing not at random (MNAR). This means the chance of seeing Y depends on Y,evenafter conditioning on X. Equivalently, f (Y X, R =0) f (Y X, R =1). MNAR is much more difficult to handle. Essentially the data cannot tell us how the missing values differ to the observed values (given X ). We are thus led to conducting sensitivity analyses. 17 / 55

18 Outline The problem of missing data and a principled approach Missing data assumptions Complete case analysis Multiple imputation Inverse probability weighting Conclusions 18 / 55

19 Complete case analysis Complete case (CC) (or complete records) analysis involves using only data from those subjects for whom all of the variables involved in our analysis are observed. CC is the default approach of most statistical packages (including Stata) when we have missing data. By only analysing a subset of records, our estimates will be less precise than had there been no missing data. Arguably more importantly, our estimates may be biased if the complete records differ systematically to the incomplete records. However, CC can be unbiased in certain situations in which the complete records are systematically different. 19 / 55

20 Validity of complete case analysis CC analysis is valid provided the probability of being a CC is independent of outcome, given the covariates in the model of interest [3]. Note that this condition has nothing to do with which variable(s) have missing values. This condition does not fit into the MCAR/MAR/MNAR classification. It is not true, as is sometimes stated, that CC is always biased if data are not MCAR! 20 / 55

21 The complete case assumption The validity of the assumption required for CC analysis to be unbiased depends on the model of interest. Returning to the example of estimating mean BP, we can think of this as the following linear model with no covariates: with ɛ i N(0,σ 2 ɛ ). BP i = α + ɛ i Here CC analysis is unbiased only of missingness is independent of BP (Y ), i.e. P(R =1 Y )=P(R =1). 21 / 55

22 Estimating mean BP - complete case analysis. reg sbp Source SS df MS Number of obs = 99 F( 0, 98) = 0.00 Model 0 0. Prob > F =. Residual R-squared = Adj R-squared = Total Root MSE = sbp Coef. Std. Err. t P> t [95% Conf. Interval] _cons The estimated mean (138.1) is biased downwards (truth=145). This is because missingness is associated with BP (higher BP more chance of BP missing). 22 / 55

23 A model for which CC is unbiased. reg sbp age Source SS df MS Number of obs = 99 F( 1, 97) = Model Prob > F = Residual R-squared = Adj R-squared = Total Root MSE = sbp Coef. Std. Err. t P> t [95% Conf. Interval] age _cons This CC analysis is unbiased, because we condition on the cause of missingness (BP). Of course this alternative model does not (by itself) give an estimate of mean BP. 23 / 55

24 Outline The problem of missing data and a principled approach Missing data assumptions Complete case analysis Multiple imputation Inverse probability weighting Conclusions 24 / 55

25 Multiple imputation Multiple imputation (MI) involves filling in each missing values multiple times. This results in multiple completed datasets. We then analyse each completed dataset separately, and combine the estimates using formulae developed by Rubin ( Rubin s rules ). By using observed data from all cases, estimates based on MI are generally more efficient than from CC. And, in some settings, MI may remove bias present CC estimates. 25 / 55

26 MI in a very simple setting There are many different imputation methods. We describe one (the classic ) in the context of a very simple setting. Suppose we have two continuous variables X and Y. X is fully observed, but Y has some missing values. Our task is to impute the missing values in Y using X. 26 / 55

27 Imputing Y from X Y ??? X 27 / 55

28 Linear regression imputation 1. Fit the linear regression of Y on X using the complete cases: where ɛ N(0,σ 2 ). Y = α + βx + ɛ 2. This gives estimates ˆα, ˆβ and ˆσ To create the mth imputed dataset: 3.1 Draw new values α m, β m and σ 2 m based on ˆα, ˆβ and ˆσ For each subject with observed X i but missing Y i,create imputation Y i(m) by: Y i(m) = α m + β m X i + ɛ i(m) where ɛ i(m) is a random draw from N(0,σ 2 m ). 28 / 55

29 The end result Data Imputation 1 Imputation 2 Imputation 3 Imputation 4 Subject Y X Y X Y X Y X Y X ? ? ? / 55

30 The analysis stage For each imputation, we estimate our parameter of interest θ, and records its standard error. e.g. θ = E(Y ), the average value of Y. Let ˆθ m and Var(ˆθ m )denotetheestimateofθ and its variance from the mth imputation. Our overall estimate of θ is then the average of the estimates from the imputed datasets ˆθ MI = M m=1 ˆθ m M where M denotes the number of imputations used. 30 / 55

31 Variance estimation The within-imputation variance is given by M m=1 Var(ˆθ m ). M This quantifies uncertainty due to the fact we have a finite sample (the usual cause of uncertainty in estimates). The between-imputation variance is given by M m=1 (ˆθ m ˆθ MI ) 2. M 1 This quantifies uncertainty due to the missing data. The overall uncertainty in our estimate ˆθ is then given by ( Var(ˆθ MI )=σw ) σb 2 M. 31 / 55

32 Inference The MI estimate and its variance can be used to form confidence intervals and performs hypothesis test. Implementations of MI in statistical packages like Stata automate the process of analysing each imputation and combining the results. 32 / 55

33 Assumptions for MI MI gives unbiased estimates provided data are MAR and the imputation model(s) is correctly specified. To be correctly specified, we must include all variables involved in our model of interest in the imputation model(s). The plausibility of MAR can be guided by data analysis and contextual knowledge. Often we have variables which are associated with missingness and the variable(s) being imputed, but which are not in the model of interest. Including these in the imputation model increases likelihood of MAR holding. 33 / 55

34 Specification of imputation models We should also ensure as best as possible that our imputation models are reasonably well specified. e.g. if a variable has a highly skewed distribution, imputing using normal linear regression is probably not a good idea. Various diagnostics can be used to aid this process, e.g. comparing distributions of imputed and observed 34 / 55

35 MI in Stata Historically the only imputation command in Stata was Patrick Royston s ice command, which performed ICE/FCS imputation (more on this later). Stata 11 included imputation using the multivariate normal model. Stata 12 adds ICE/FCS imputation functionality. 35 / 55

36 Imputing missing BP values in Stata Step 1 - mi set the data e.g. mi set wide Alternatives include mlong, flong. This only affects how Stata organises the imputed datasets. 36 / 55

37 Imputing missing BP values in Stata Step 2 - mi register variables At a minimum, we must mi register variables with missing values we want to impute. e.g. mi register imputed sbp 37 / 55

38 Imputing missing BP values in Stata Step 3 - imputing the missing values We are now ready to impute the missing values. Since we have only missing values in one continuous variable, we shall impute using a linear regression imputation model:. mi impute reg sbp age, add(10) rseed(5123) Univariate imputation Imputations = 10 Linear regression added = 10 Imputed: m=1 through m=10 updated = 0 Observations per m Variable Complete Incomplete Imputed Total sbp (complete + incomplete = total; imputed is the minimum across m of the number of filled-in observations.) 38 / 55

39 Imputing missing BP values in Stata Step 4 - analysing the imputed datasets We are now ready to analyse the imputed datasets. This is done by Stata s mi estimate command, which supports most of Stata s estimation commands.. mi estimate: reg sbp Multiple-imputation estimates Imputations = 10 Linear regression Number of obs = 200 Average RVI = Largest FMI = Complete DF = 199 DF: min = avg = DF adjustment: Small sample max = F( 0,.) =. Within VCE type: OLS Prob > F =. sbp Coef. Std. Err. t P> t [95% Conf. Interval] _cons The estimate is quite close to the true value (145). 39 / 55

40 Other MI imputation methods in Stata In addition to linear regression Stata s mi command offers imputation using: Logistic, ordinal logistic, and multinomial logsitic models Predictive mean matching Truncated normal regression for imputing bounded cts variables Interval regression for imputing censored cts variables Poisson regression for imputing count data Negative binomial regression for imputing overdispersed count data 40 / 55

41 MI with more than one variable So far we have considered setting with one variable partially observed. Often we have datasets with multiple partially observed variables. Stata 11/12 supports imputation with the multi-variate normal model. What if we have categorical or binary variables with missing values? More on this in tomorrow s course / 55

42 Outline The problem of missing data and a principled approach Missing data assumptions Complete case analysis Multiple imputation Inverse probability weighting Conclusions 42 / 55

43 Inverse probability weighting Inverse probability weighting (IPW) for missing data takes a different approach [4]. We perform a CC analysis, but weight the complete cases by the inverse of their probability of having data observed (i.e. not being missing). Those who had a small chance of being observed are given increased weight, to compensate for those similar subjects who are missing. This requires us to model how missingness depends on fully observed variables. 43 / 55

44 Using IPW to estimate mean BP Recall our previous analysis of missingness in BP and age:. tab age r, chi2 row Key frequency row percentage r age 0 1 Total Total Pearson chi2(1) = Pr = The probability of observing BP is 0.72 for year olds, and 0.27 for year olds. So the weight for year olds is 1/0.72 = 1.39 and for year olds is 1/0.27 = / 55

45 The IPW estimator Since we are interested in estimating a simple parameter (mean BP), we can manually calculate the IPW estimate: =145.1 IPW appears has removed the bias from the simple CC estimate of mean BP. 45 / 55

46 IPW more generally Step 1 - Constructing weights With multiple fully observed variables, we can use logistic regression to model missingness:. logistic r age Logistic regression Number of obs = 200 LR chi2(1) = Prob > chi2 = Log likelihood = Pseudo R2 = r Odds Ratio Std. Err. z P> z [95% Conf. Interval] age _cons predict pr, pr. gen wgt=1/pr 46 / 55

47 IPW more generally Step 2 - parameter estimation We can then pass the constructed weights to our estimation command:. reg sbp [pweight=wgt] (sum of wgt is e+02) Linear regression Number of obs = 99 F( 0, 98) = 0.00 Prob > F =. R-squared = Root MSE = Robust sbp Coef. Std. Err. t P> t [95% Conf. Interval] _cons Notice that the SE is larger (2.16) compared to the MI SE (1.75). 47 / 55

48 Outline The problem of missing data and a principled approach Missing data assumptions Complete case analysis Multiple imputation Inverse probability weighting Conclusions 48 / 55

49 Problems caused by missing data and a principled approach Missing data reduce precision and potentially parameter bias estimates and inferences. Producing valid estimates requires additional assumptions about the missingness to be made. Ad-hoc methods should generally be avoided. Both data analysis and contextual knowledge should guide us in thinking about missingness in a given setting. We can then choose a statistical method which accommodates missing data under our chosen assumption (e.g. MAR). 49 / 55

50 Complete case analysis Complete case (CC) analysis is the default method of most software packages, including Stata. CC analysis is generally biased unless data are MCAR. But it can be unbiased in certain non-mcar settings when the model of interest is a regression model. Even when it is unbiased, CC may be inefficient compared to other methods. 50 / 55

51 Multiple imputation Multiple imputation is a flexible approach to handling missing data under the MAR assumption [5]. Stata 12 now includes a comprehensive range of MI commands, including ICE/FCS MI. In settings where both CC and MI are unbiased, MI will generally give more precise estimates. We must carefully consider the plausibility of the MAR assumption and whether imp. models are correctly specified. 51 / 55

52 Inverse probability weighting IPW involves performing a weighted CC analysis. Rather than model the partially observed variable, we model the observation/missingness indicator R. The weights based on this model are then passed to our estimation command, and most Stata estimation commands support weights. Sometimes modelling missingness may be easier than modelling the partially obs. variable (e.g. if the partially observed variable has a tricky distribution). However, IPW estimators can be quite inefficient compared to MI or maximum likelihood. IPW is also difficult (or impossible) to use in settings with complicated patterns of missingness. 52 / 55

53 Sensitivity to the MAR assumption Since we can never definitively our assumptions (e.g. MAR) hold, we should consider sensitivity analysis. MI can also be used to perform MNAR sensitivity analyses [6]. If you want to learn more, come on our missing data short course at LSHTM in June. And/or visit our website 53 / 55

54 References I [1] L. Marston, J. R. Carpenter, K. R. Walters, R. W. Morris, I. Nazareth, and I. Petersen. Issues in multiple imputation of missing data for large general practice clinical databases. Pharmacoepidemiology and Drug Safety, 19: , [2] D B Rubin. Inference and missing data. Biometrika, 63: , [3] I. R. White and J. B. Carlin. Bias and efficiency of multiple imputation compared with complete-case analysis for missing covariate values. Statistics in Medicine, 28: , / 55

55 References II [4] S. R. Seaman and I. R. White. Review of inverse probability weighting for dealing with missing data. Statistical Methods in Medical Research, [5] J A C Sterne, I R White, J B Carlin, M Spratt, P Royston, M G Kenward, A M Wood, and J R Carpenter. Multiple imputation for missing data in epidemiological and clinical research: potential and pitfalls. British Medical Journal, 339: , [6] J R Carpenter, M G Kenward, and I R White. Sensitivity analysis after multiple imputation under missing at random a weighting approach. Statistical Methods in Medical Research, 16: , / 55

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