Rod:
Sorry, I have to side with Seppo Laaksonen on this one. I teach missing
data handling to applied social scientists as part of my structural
equation modeling workshops. In my experience, the MAR-MCAR distinction
is confusing at best. For non-statisticians, these two terms simply
*sound* too similar to make a meaningful distinction. In addition,
people's common-sense understanding of the term "missing at random"
is really MCAR. This is reflected in Seppo's desire to change the
"MAR" terminology to "missing at random conditionally." In other words,
the MAR-MCAR terminology is somewhat of an obstacle to teaching
ML-based missing data methods to non-statisticians.
I do not mean to diminuish Don Rubin's eminent contributions to the
missing data field in any way--quite the contrary. On the other hand,
after 27 years of MAR-MCAR it is really time to consider replacing that
terminology with one more easily understood by applied researchers.
Yours and Don's assistance in this matter would help a lot.
Cheers,
Werner Wothke, Ph.D.
Rod Little wrote:
>
> Dear Seppo: you are not the first to make such a comment. I'd say that MAR
> is one of the few terms in the area I like, since it at least has a
> well-defined meaning! It was originally coined in Rubin's 1976 Biometrika
> paper, and the rationale was that people were doing analyses ignoring the
> missing data mechanism that had an implied assumption, and he defined MAR
> to be that implied assumption. For likelihood inference that assumption is
> MAR. Since there is another reasonable term for the "randomness" you are
> talking about, namely missing completely at random, I see no real problem.
> There is an strong argument that people who write the seminal article
> should get to define terms, and I think trying to change a definition is
> asking for confusion.
>
> Concerning other terms, is there an established definition of
> "informatively missing"? I'd be interested in comments on this. Best, Rod
>
> On 29 Mar 2001, Laaksonen Seppo wrote:
>
> > I do not like about the term MAR, missing at random. Of course, when it has
> > been defined, there are no problems. But the direct interpretation of that term
> > is confusing, since missingness is not random in this case but conditionally in
> > some sense. The term should be something like missing at random conditionally
> > (MARC) or MAR according to covariates. I am not fully satisfied to those terms.
> > What do you prefer?
> >
> > Best regards
> > Seppo Laaksonen
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