  Field-normalization of citations is bibliometric standard. Despite the
observed differences in citation counts between fields, the question remains
how strong fields influence citation rates beyond the effect of attributes or
factors possibly influencing citations (FICs). We considered several FICs such
as number of pages and number of co-authors in this study. We wondered whether
there is a separate field-effect besides other effects (e.g., from numbers of
pages and co-authors). To find an answer on the question in this study, we
applied inverse-probability of treatment weighting (IPW). Using Web of Science
data (a sample of 308,231 articles), we investigated whether mean differences
among subject categories in citation rates still remain, even if the subject
categories are made comparable in the field-related attributes (e.g.,
comparable of co-authors, comparable number of pages) by IPW. In a diagnostic
step of our statistical analyses, we considered propensity scores as covariates
in regression analyses to examine whether the differences between the fields in
FICs vanish. The results revealed that the differences did not completely
vanish but were strongly reduced. We received similar results when we
calculated mean value differences of the fields after IPW representing the
causal or unconfounded field effects on citations. However, field differences
in citation rates remain. The results point out that field-normalization seems
to be a prerequisite for citation analysis and cannot be replaced by the
consideration of any set of FICs in citation analyses.
