Effect of instrument precision on estimation of low birth weight prevalence.
OBJECTIVE
Reductions in prevalence of low birth weight (LBW) are likely to be achieved only through small increments, amplifying the importance of precision of measurement. This study compared two instruments for measuring birth weight to investigate the effect of instrument precision on estimates of LBW prevalence.


METHODS
Analysis was based on 497 infants born to mothers enrolled in an ongoing community-based trial of the effect of umbilical cord and skin antisepsis on neonatal mortality in Nepal. For each child, two birth weight measurements were recorded within 72 hours after delivery using weighing scales of differing precisions (100 vs 2 g).


RESULTS
While continuous measures between the two instruments were similar, the prevalence of LBW among lower precision measurements (30%) was 11.3% lower than for higher precision measurements (34%). The difference in precision between the instruments accounted for 96% of the difference in LBW prevalence estimates.


CONCLUSIONS
Differences in estimated LBW rates may be entirely due to differences in instrument precision. Conclusions concerning programmatic or research intervention impact, or comparisons of rates across populations should consider the effect of instrument precision on estimates of LBW.
Mullany
Luke C
LC
Department of International Health, Bloomberg School of Public Health, Baltimore, MD 21205, USA.
Darmstadt
Gary L
GL
Katz
Joanne
J
Khatry
Subarna K
SK
Tielsch
James M
JM
D000886:Anthropometry / Q000295:instrumentation*; D001724:Birth Weight; D006801:Humans; D007230:Infant, Low Birth Weight; D007231:Infant, Newborn; D009390:Nepal; D015995:Prevalence; D012680:Sensitivity and Specificity; D013223:Statistics as Topic