SAS propensity score matching: Observations considered for matching in PSMATCH is less than the total observations available in the data set

Question

I am using SAS procedure PSMATCH to balance the cohorts. I am calculating the propensity score separately using logistic regression and then using the generated dataset in PSMATCH using PSDATA. I am doing multiple iterations of matching (to get the best results) by bringing variation in region, method (Optimal, Greedy and variable ratio), distance variable, caliper value and ratio. Please find the code below:

proc psmatch data=work.&data_set. region=&region_var.;
    class &cat_var.;
    psdata treatvar = case_cntrl_fl(Treated='1') PS=prop_score;
    match method=&mtch_method.(&k_method.=&k_val.) exact= &.exact_mtch_var. 
    stat=&stat_var. caliper(mult=stddev)=&caliper_var.;
    assess lps ps var=(prop_score &covar_asses.) / plots = (boxplot cloudplot);
    output out(obs=match)=WORK.psm ps=ps lps=lps matchid=_MatchID matchwgt = _MATCHWGT_;
run;

My concern is regarding the number of observation considered for matching (ie All Observations). The total observation logistic regression data set are Treatment Arm 1: 531 and Treatment Arm 2: 3252 However, in PSMATCH report All observations reported as Treatment Arm 1: 446 and Treatment Arm 2: 2784 The result is consistent irrespective of the variations in PSMATCH methods

Can somebody help me understand the possible reason of drop in counts?

Answer 1

You likely have missing values in your data. If any variable in the proc is missing, that entire row is excluded from the analysis overall.