Hi, I wanted to conduct an ANOVA to compare the mean difference between groups. Firstly, I used the below PROC GLM to check the equal variance assumption, and the result was significant, since the sample size per cell was not that big, I used the Obrien.
proc glm data=one;
class region;
model cost=region;
means region/hovtest=OBRIEN welch ;
run;
And the Welch test showed the result was significant, so, in order to account for the unequal variance, I used the below code to conduct the post-hoc comparison.
proc mixed;
class region;
model cost=region/ddfm=satterth;
repeated / group=region;
lsmeans region / pdiff;
run;
I checked the SAS website, https://support.sas.com/kb/22/526.html, it says that group=region in the repeated statement will allow SAS to estimate the variance separately, and ddfm=satterth will adjust the degree of freedom. However, the least square mean result showed that two groups were significant, but in the difference least square mean result, there is no significant result found. I tried to use different adjust=, but the result was the same.
I saw the note below, probably it is because the Hessian is not positive definite? Is there anything else that I should do to address this issue? Thank you so much!
NOTE: Convergence criteria met but final Hessian is not positive definite.
NOTE: Asymptotic variance matrix of covariance parameter estimates has been found to be singular and
a generalized inverse was used. Covariance parameters with zero variance do not contribute to
degrees of freedom computed by DDFM=SATTERTH.
