i would like to add one thing.
do more research on schools.
i mean when preparing to apply, applicants must consider not only the research fit, but also the grad school's general atmosphere.
i had overlooked this point, and based on what i saw, i thought that many applicants also did not care much about this.
although research productivity may be high enough equivalently, some schools focus more on theoretical advancement itself, while others pursue the knowledge for practice or for contribution to the society. of course there cannot be a clear distinction, but their emphases do differ.
to be admitted, i believe that applicants should show good fit with schools' emphasis, in addition to the perfect research fit.
if i had been more confident about this, i would have saved some money.