>>924
https://www.adamsmith.org/blog/welfare-pensions/the-negative-income-tax-and-basic-income-are-pretty-much-the-same-thing

これが出どころということになるかと思いますが
ここでいうwithdraw rateとは負の所得税が所得に応じてどれほど減額されるかという率を指すようです

具体的にはここですね

An example: a £5,000 basic payment at a 50% marginal withdrawal rate (this means
that for every additional pound earned, the worker will receive 50p less in NIT
payments). Someone with an income of zero would receive an NIT payment of
£5,000, or just under £100/week. If they took a job that paid £5,000/year, they
would receive a top-up of £2,500/year; that paid £7,500, a top-up of
£1,250/year. Once they reached £10,000/year, they would receive nothing in NIT.

貯蓄・投資とかの引き出し率とは別ですね。