  The metadata about scientific experiments are crucial for finding,
reproducing, and reusing the data that the metadata describe. We present a
study of the quality of the metadata stored in BioSample--a repository of
metadata about samples used in biomedical experiments managed by the U.S.
National Center for Biomedical Technology Information (NCBI). We tested whether
6.6 million BioSample metadata records are populated with values that fulfill
the stated requirements for such values. Our study revealed multiple anomalies
in the analyzed metadata. The BioSample metadata field names and their values
are not standardized or controlled--15% of the metadata fields use field names
not specified in the BioSample data dictionary. Only 9 out of 452
BioSample-specified fields ordinarily require ontology terms as values, and the
quality of these controlled fields is better than that of uncontrolled ones, as
even simple binary or numeric fields are often populated with inadequate values
of different data types (e.g., only 27% of Boolean values are valid). Overall,
the metadata in BioSample reveal that there is a lack of principled mechanisms
to enforce and validate metadata requirements. The aberrancies in the metadata
are likely to impede search and secondary use of the associated datasets.
