Systematic Bias (Built-in Error) in MUAC Tapes With Varying Thickness
Type A Tape (with material thickness of 180 microns) | Type B Tape (with material thickness of 280 microns) |
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Example 1: On a 100 mm true circumference, we have: D = C/π = 100/3.14159 = 31.8310 | |
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C = π × (31.8310 + 2a × 0.18) = 101.131 mm Built-in error of about 1.13 mm |
C = π × (31.8310 + 2a × 0.28) = 101.759 mm Built-in error of about 1.76 mm |
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Example 2: If we consider a 200 mm circumference, we have: D = C/π = 200/3.14159 = 63.6620 | |
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C = π × (63.6620 + 2a × 0.18) = 201.131 mm Built-in error also 1.13 mm |
C = π × (63.6620 + 2a × 0.28) = 201.759 mm Built-in error also 1.76 mm |
Abbreviations: D, diameter; C, circumference; π (pi); MUAC, mid-upper arm circumference.
↵aNeed to add twice the thickness of the material (For more detailed explanation of why the error is constant despite different circumferences, see the “String Girdling Earth” mathematical puzzle).