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研究对象未按计划被随访很可能造成的偏倚是（）
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某男性工人，40岁，是某家具厂的油漆工，工龄约10年。近一段时间感觉头痛、头晕和失眠等症，易患感染并常伴有牙龈和皮下出血。 应采用何种治疗方法
维生素D缺乏症是（）
Profits grew steadily in the three month.
A．Flat Tax Income tax is a direct tax which is levied on the income of private individuals．There are various income tax systems that exist．ranging frOm a flat tax to an extensive progressive tax systern． A flat tax，also cal led a proportional tax，is a system that taxes．Usual ly the flat tax is proposed to k ick in at a certain income level，or to exempt income below that level，so that the lowest-income members of society don’t need to pay income tax． Proposed flat taxes usually allow little or no exemption of earned income besides the bottom-level exemption． Advocates of a flat tax claim that it will end unfair discrimination．They also argue that flat taxes are easier(and cheaper)to administer and comply with than complex，graduated taxes．Most political parties that advocate the introduction of a flat tax are on the right of the pol itical spectrum．B．Progressive Tax A progressive tax，or graduated tax，is a tax that is larger as a percentage of income for those with larger incomes．It is usually applied in reference to income taxes，where people with more income pay a higher percentage of it in taxes．The term progressive refers to the way the rate progresses from low to high．C．Regressive Tax A regressive tax is a tax which takes a larger percentage of income from people whose income is low． A tax which places placesonately more of a burden on those with lower incomes． Regressive taxes，as opposed to progressive taxes，are more burdensome on lower-income individuals than on higher-income individuals and corporations．D．Tax Deduction Within the United States’income tax system，a tax deduction，or" tax-deductible expense" ，is an item which is subtracted from gross income in order to arrive at the taxable income． Effectively，the taxpayer pays no income tax on the amount of money he spent on tax-deductible expenses．For example．if an individual earns$50，000 in a year and gives $5，000 to tax-deductible charities，he will end up paying income tax as though he had earned only$45，000 that year．In this way，the federal and state governments encourage certain types of spending．E．Tax Avoidance and Tax Evasion Tax avoidance is the legal exploitation of the tax regime to one’s own advantage，to attempt to reduce the amount of tax that is payable by means that are within the law whilst making a full disclosure of the materialinformation to the tax authorities． Tax evasion，on the other hand，is a crime．Tax evasion usually entails taxpayers deliberately misrepresenting or concealing the true state of their affairs to the tax authorities to reduce their tax liability，and includes，in particular，dishonest tax reporting，such as under-declared income，profits or gains；or overstated deductions． This tax forces lower-income people and corporations to earn more money，because the less you earn，the more taxes you have to pay．
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