Individual Blood Lead Measurement is the Only Valid Method
None of the statistical measures listed (mean, mode, median, or standard deviation) can assess the degree of lead exposure for each individual child—only direct venous blood lead measurement of each child can accomplish this. 1, 2, 3
Why Statistical Measures Cannot Assess Individual Exposure
Understanding the Question's Fundamental Error
Statistical measures describe group characteristics, not individual values. The mean, mode, median, and standard deviation are summary statistics that characterize a population or sample as a whole, but they provide no information about any specific individual's exposure level. 1
Each child requires their own blood lead measurement because lead exposure varies dramatically between individuals even within the same high-risk area, depending on specific home conditions, behaviors (hand-to-mouth activity), renovation status, and other individual factors. 1
Venous blood lead testing is the gold standard for assessing an individual's current lead exposure and body burden, and must be performed on each child separately. 2, 3
The Correct Approach to Individual Assessment
Direct Measurement Requirements
Obtain venous blood samples from each of the 50 children individually using OSHA-designated laboratories that meet specific proficiency requirements, ideally achieving performance within ±2 μg/dL rather than the federally permitted ±4 μg/dL. 2, 3
Avoid capillary (fingerstick) samples for diagnostic purposes as they are prone to contamination from lead on the skin surface; if used for initial screening, any elevated result must be confirmed with venous blood. 2, 3
Each child's blood lead level represents their individual degree of exposure, with concurrent blood lead levels (measured at a specific time) providing the most accurate assessment of current exposure status. 1
Why This Question May Be Testing Statistical Literacy
If this is an examination question testing understanding of statistical concepts rather than clinical practice:
Standard deviation (Option D) describes the spread or variability of lead levels across the group of 50 children, but still does not assess any individual child's exposure. 1
The mean (Option A) would give the average exposure across all 50 children, potentially masking dangerously high individual exposures. 1
The median (Option C) identifies the middle value when all 50 measurements are ordered, but tells nothing about individual children's specific exposures. 1
The mode (Option B) identifies the most frequently occurring value, which is clinically meaningless for lead exposure assessment. 1
Critical Clinical Context
Individual Variability in High-Risk Areas
Even among high-risk children with initial blood lead levels <10 μg/dL at age 1 year, 21% developed levels >10 μg/dL when retested after age 2 years, demonstrating substantial individual variation over time within the same population. 4
Lead exposure changes with developmental progress (walking, reaching window sills) and external factors (home remodeling, family relocation), making individual assessment essential. 1, 4
No safe threshold exists for lead exposure—even levels <5 μg/dL are associated with decreased IQ, academic achievement, and neurodevelopmental problems, with greater intellectual decrements per unit increase at lower blood lead levels. 4, 5
The Dose-Response Relationship
Environmental lead exposure in children with maximal blood lead levels <7.5 μg/dL is associated with greater intellectual deficits per unit increase compared to children with higher baseline exposures, emphasizing the importance of identifying each individual's specific exposure level. 5
An increase in blood lead from 2.4 to 10 μg/dL is associated with a 3.9 IQ point decrement (95% CI, 2.4-5.3), while the same magnitude increase from 20 to 30 μg/dL produces only a 1.1 point decrement, demonstrating non-linear effects that vary by individual baseline exposure. 5