This study is a prospective cohort
study, and the main exposure factor is BVHS. BVHS is an ordered
classification variable, taking the median of BVHS as the median, we
regard the higher BVHS as the
exposure group and the lower as the control group. The sample size of
the cohort study was calculated by using the sample size formula of the
cohort study when the sample size of the exposure group was equal to
that of the control group:
,
α indicates the significant level, set to 0.05; 1-β indicates the test
efficacy, set to 0.9; p0 indicates the incidence of the
control group;
;
p1 indicates the incidence for the exposure group,
;
,;
The incidence of natural population events was taken as the estimation
of the incidence of the control group, that is
,
and the risk ratio (RR) value of the exposure group was used to estimate
the incidence of the exposure group. According to the previous research
results of our group10, the RR value was estimated to
be 1.94,
.
Under the above parameters, the minimum sample size is 1471.