There was a recent discussion here of errors in journal articles. A
related topic is incomplete information, or at least what I consider
incomplete information.
A recent article in the American Journal of Epidemiology (2001, Vol 153,
No 6, 596-603) contains some nicely laid out (and badly titled, but save
that for another thread) tables that show the mean value of a dependent
variable in male and female subsamples, broken down by a dichotomous
independent variable (exercise? Yes/No), and "adjusted" first by age
and then by age and several other numerical variables.
In a sociological journal I would most often see this analysis reported
in multiple regression form, but again, that's another thread. In health
related journals the convention is to speak of adjusted means, i.e. the
predicted dependent variable value for members of each category of the
independent variable, with the other predictors ("covariates") set to
specific values.
The article does not specify what values of age (and the other
predictors) were used to create the adjusted means. Instead there is a
footnote to the table that says:
"Adjusted means calculated by using analysis of covariance."
My question, directed to those of you who are more familiar with
journals in this area than I am, is whether this is a standard footnote
/ explanation, which is supposed to make clear to regular readers what
has been done?
>From my perspective it is inadequate, since the ANCOVA (or regression
analysis) has merely produced a predictive formula, and any values
whatsoever of the covariates could be plugged in to the equation.
Now I happen to know what SPSS v10 does when asked to produce
"estimated" means in its univariate GLM procedure: it plugs in the mean
values. The output actually contains the values of those means for the
record. (Is this true with other statistical software?) A user who knows
the syntax can actually specify the values, but the Windows point+click
screen doesn't allow that. Here's an example of the default subcommand
statement:
/EMMEANS = TABLES(exercise) WITH(age=MEAN xother=MEAN)
Using the mean can produce misleading adjusted values, especially when
the table contains subsample comparisons as in this article, where "all
analyses were sex-specific". If the default SPSS ANCOVA were followed,
the adjusted values would be created at different values of age, since
mean age differs by sex, and other covariate means are also different in
this study. (In addition, different tables contained analyses of 2 other
subgroups of the sample, with different mean Xs.)
This may or may not be a problem for the authors' interpretation of the
results, but it seems reasonable to expect editors to be more sensitive
to their readers' need to know exactly what is going on.
--
******************************************
`o^o' * Neil W. Henry ([EMAIL PROTECTED]) *
-<:>- * Virginia Commonwealth University *
_/ \_ * Richmond VA 23284-2014 **
* http://www.people.vcu.edu/~nhenry *
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