This study examines whether the Bayley Scales of Infant Development-Second
Edition (Bayley II) Mental Scale scores vary on the basis of which item set
is considered the starting point of an infant's assessment. The Bayley II
was administered to 78 12-month-old infants by certified examiners beginnin
g with the 12-month age item set. A second certified examiner then administ
ered 10 additional items that completed the Ii-month and 13-month age item
sets. Of the 78 infants tested, 73 (94%) met basal and ceiling criteria in
all three item sets. Three separate Mental Developmental Index (MDI) scores
were calculated using le-month norms for each subject, which were based on
the raw scores generated from the 11-, 12-, and 13-month age item sets. Sc
ores calculated on the Ii-month set were significantly lower than those on
the 12-month set, which were in turn significantly lower than those on the
13-month set. When tested on the Ii-month instead of on the 12-month item s
et, twice as many infants received lower than normal (<85) MDI scores, indi
cating an impact on referability decisions. Minor adjustments in administra
tion of the Bayley II, such as those based on assumptions regarding an infa
nt's current level of functioning, significantly affect infant test scores
and eligibility for follow-up services. Standardized use of this test shoul
d minimize variability in test scores.