Later parametric modeling using the 1984 SIPP was influential in highlighting the importance of length bias
(discussed below) when the analytic sample includes uninsured spells already in progress at the beginning of the survey (Swartz, McBride, and Marcotte 1993).
Because the length bias
of the shotgun sequencing reads was suspected to derive from the PCR, one could potentially ligate universal adaptors to the ends of plasma DNA and then perform PCR amplification against the universal sequences to enrich the fetal DNA fraction.
This study is subject to more serious biases that are associated with non-randomized screening studies: lead-time bias and length bias
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