Statistical analyses
We calculated mean and median payments per neurologist and proportion of
neurologists receiving payments. We assessed the concentration of
payments among neurologists using the Gini index, a measure
traditionally applied to analyze income inequality in economics[53].
The index ranges from 0 (indicating uniform payment distribution) to 1
(where a single neurologist receives all payments), with higher values
signaling greater disparity. We also analyzed payment data by category
and the pharmaceutical companies making these payments. Trends in the
number of neurologists receiving payments and the payment amounts from
2016 to 2020 were evaluated using generalized estimating equation (GEE)
models. Due to the non-normal distribution of payments, the analyses
were conducted using a log-linked GEE model with a Poisson distribution
and a negative binomial GEE model[20-23]. The study period was
divided into two intervals (2016-2019 and 2020) to evaluate impact of
COVID-19 pandemic on the payment patterns, as indicated in previous
studies in the United States[33,35-37,39]. For trend analysis, we
adjusted for inflation, converting all payment values to 2020-Japanese
yen value with consumer price index. Statistical significance was set at
a p-value of less than 0.05.