2.3 IVs selection criteria
SNPs associated with the exposures at genome-wide significance (P
< 5.0×10-8) were selected as
IVs. However, this criterion was too
strict for mixed asthma: only one SNP could be used as the IV, so we set
a P value of less than 5.0×10-6 for mixed asthma only.
Then, independent IVs were identified utilizing the cutoff of the
matching linkage disequilibrium (LD) value
(threshold set at
r2 <
0.001 and clump window > 10,000 kb) to ensure their
independence. We further removed SNPs
associated with the outcomes (P<5.0×10-5)
and SNPs associated with confounders via the PhenoScannerV2 database
(http://www.phenoscanner.medschl.cam.ac.uk/). To assess the strength of
IVs, the F statistic was calculated for each IV: F statistic =
R2 (N-2)/(1-R2),
R2: the phenotypic variance explained by each IV in
the exposure, N: the sample size14,15. IVs with an F
statistic < 10 were considered weak and should be
deleted16.