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Date:         Thu, 22 Dec 2011 17:36:19 -0500
Reply-To:     Sas Beginner <cep@STUNTZ.ORG>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Sas Beginner <cep@STUNTZ.ORG>
Subject:      creating propensity score. quasi separation error


i created a dataset of 6-month time blocks. within each 6-month block, i am running proc logistic to create propensity scores. i then use greedy matching to match the exposed subjects to reference subjects (within this 6-month block).

it was all looking good until i hit a warning on one of the 6-month blocks.

here is my sas code. PROC LOGISTIC DATA=inpatient_229 Descend; MODEL initiator = age alcoholism_prior beta_blocker_2 bis_2 bmi_prior diuretics_2 dvt_prior estrogen_2 gc_2 heart_failure_prior high_fall_risk_prior hyperthyroidism_prior neuro_impair_prior pul_emb_prior smoking_prior statins_2 /SELECTION = STEPWISE RISKLIMITS LACKFIT RSQUARE PARMLABEL; OUTPUT OUT= Propen229 prob=prob; RUN;

here is the sas log:

NOTE: PROC LOGISTIC is modeling the probability that initiator=1. WARNING: There is possibly a quasi-complete separation of data points. The maximum likelihood estimate may not exist. WARNING: The LOGISTIC procedure continues in spite of the above warning. Results shown are based on the last maximum likelihood iteration. Validity of the model fit is questionable. NOTE: There were 3484 observations read from the data set V20FRAC.INPATIENT_229. NOTE: The data set WORK.PROPEN229 has 3484 observations and 81 variables.

my questions: - is there anything i can do to resolve the warning? - should i just not include this specific 6-month block in my final analysis dataset?

many thanks! happy holidays.

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