The principles of balance, randomization, control and repetition, which are closely related, constitute the four principles of scientific research. The balance principle is the kernel of the four principles which runs through the other three. However, in scientific research, the balance principle is always overlooked. If the balance principle is not well performed, the research conclusion is easy to be denied, which may lead to the failure of the whole research. Therefore, it is essential to have a good command of the balance principle in scientific research. This article stresses the definition and function of the balance principle, the strategies and detailed measures to improve balance in scientific research, and the analysis of the common mistakes involving the use of the balance principle in scientific research.

Randomization is one of the four basic principles of research design. The meaning of randomization includes two aspects: one is to randomly select samples from the population, which is known as random sampling; the other is to randomly group all the samples, which is called randomized grouping. Randomized grouping can be subdivided into three categories: completely, stratified and dynamically randomized grouping. This article mainly introduces the steps of complete randomization, the definition of dynamic randomization and the realization of random sampling and grouping by SAS software.

The control principle is one of the four basic principles of research design. Without a control group, the conclusion of research will be unconvincing; furthermore, if the control group is not set properly, the conclusion will be unreliable. Generally, there is more than one control group in a multi-factor design. Problems like incomplete control and excessive control should be avoided. This article introduces the meaning and function of the control principle, common forms of control, common errors that researchers tend to make as well as analysis and differentiation of these errors.

The repetition principle is important in scientific research, because the observational indexes are random variables, which require a certain amount of samples to reveal their changing regularity. The repetition principle stabilizes the mean and the standard variation, so that statistics of the sample can well represent the parameters of the population. Thus, the statistical inference will be reliable. This article discussed the repetition principle from the perspective of common sense and specialty with examples.

Scientific research design includes specialty design and statistics design which can be subdivided into experimental design, clinical trial design and survey design. Usually, statistics textbooks introduce the core aspects of experimental design as the three key elements, the four principles and the design types, which run through the whole scientific research design and determine the overall success of the research. This article discusses the principle of randomization, which is one of the four principles, and focuses on the following two issues--the definition and function of randomization and the real life examples which go against the randomization principle, thereby demonstrating that strict adherence to the randomization principle leads to meaningful and valuable scientific research.

The research subject is the first key element of the three key elements in the research design. An appropriate selection of research subjects is crucial to the success of the research. This article summarizes the general principles for the selection of research subjects, the types and numbers of research subjects and the common mistakes that researchers tend to make in the selection of the research subjects. This article also provides the methodology suggestions for the selection of research subjects.

Objective To explore the dose- and time-responses of BPCBG on the decorporation of uranium and its protective effects for uranium-induced kidney injury in rats. Methods Sprague-Dawley (SD) male rats were randomly divided into 4 -7 groups:normal control group,uranium poisoning group,different doses of BPCBG groups and DTPA-CaNa3 group. Rats in chelating agents-treated groups were either injected intramuscularly with 60,120 and 600 μmol/kg of BPCBG or 120 and 600 μmol/kg of DTPA-CaNa3 immediately after intraperitoneal injection of uranyl acetate dihydrate,or injected with 120 μmol/kg of BPCBG 0.5,2 h before or 0,0.5,1 and 2 h after injection of uranium. Uranium poisoning group rats were injected with normal saline after intraperitoneal injection of uranyl acetate dihydrate,and the normal control group rats were merely injected with normal saline. The uranium content in urine,kidney and femurs were detected 24 h after chelator injections by ICP-MS method.After injecting a dose of 500 μg uranyl acetate dihydrate,rats were injected with 600 μmol/kg of BPCBG or 1200 μmol/kg of DTPA-CaNa3. Histopathological changes in the kidney and serum creatinine and urea nitrogen were examined 48 h after chelator administration.Results Prompt injections of BPCBG resulted in 37％ -61％ ( t =2.22,4.43,5.80,P ＜ 0.05 ) increase in 24 h-urinary uranium excretion,and significantly decreased the levels of uranium in kidney and bone by 59％ -69％ (t=3.33,5-59,4-53,P＜0.01) and 14％ -58％ (t =2.15,8.70,9.10,P ＜ 0.05 ) respectively in a dose-dependent manner. BPCRG injection obviously reduced the severity of the uranium-induced histological alterations in the kidney,which was in parallel with the amelioration noted in serum indicators,serum creatinine and urea nitrogen,of uranium nephrotoxicity.Advanced 0.5 h or delayed 0.5 and 1 h administrations of BPCBG were effective in 24 h-urinary uranium excretion ( advanced 0.5 h:t =4.34,delayed 0.5 h:t =3.35,P ＜ 0.05 ),decreasing accumulation of kidney uranium ( t =5.75,7.74,5.87,P ＜ 0.05 ) and accumulation of hone uranium (t =6.43,5.222,2.60,P ＜0.05),but the efficacy decreased with the interval time between uranium and BPCBG injection. Although DTPA-CaNa3 markedly reduced uranium retention in kidney (120,600 μmol/kg,t =2.28,3.35,P ＜ 0.05 ),its efficacy in uranium removal was significantly lower than that of BPCBG,and it had no protective effects against uranium-induced nephrotoxicity.Conclusions BPCBG can effectively decorporate uranium from rats and protect against uranium-induced kidney injury of rats.

Research factors are a very important element in any research design. Research factors include experimental and non-experimental factors. The former is the general term used to describe the similar experimental conditions that researchers are interested in, while the latter are other factors that researchers have little interest in but may influence the result. This article mainly focuses on the following issues: the definition of research factors, the selection and arrangement of experimental factors and non-experimental factors, the interaction between research factors, the standardization of research factors and the common mistakes frequently made by researchers.

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