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.

The transitional near-infrared (NIR) laser was defined as ranging from 1.3μm to 1.4μm, within which the most sensitive tissue to laser damage changed from the retina to the cornea.The ocular damage effect has attracted much attention due to the increased varieties and output power of laser in this spectrum region in recent years.Compared with visible and mid-and-far infrared wavelengths, the ocular damage effect induced by transitional NIR wavelengths has many peculiarities and impact factors due to the bulk absorptionby ocular media.This paper reviews the existing ocular damage threshold data and analyzes the characteristics, impact factors and unresolved issues relating to ocular effects induced by laser radiation over the transition zone.

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.

Objective To study the molecular genetic mechanism of a patient with 17? hydroxylase (CYP17) deficiency. Methods Genomic DNA were abstracted from the blood of the patient, her parents and healthy control. The 8 exons of CYP17 gene were amplified, using 5 pairs of designed primers, with polymerase chain reaction (PCR), and the 8 exons were sequenced by the dideoxy terminator method to determined the mutation sites. The corresponding exons of the parents of the patients were also amplified and sequenced to determine the zygosity of the patient and the source of the gene variances. Results The analysis revealed that the patient (46, XY) was a compound heterozygote carrying two different inherited mutations on CYP17 gene, one from mother containing a point mutation Arg 96 (C G G)→ Gln(C A G) and the other from father containing a nine base deletion (CACTCTTTC) at amino acid position 487～489 (Asp Ser Phe) near the carboxyl terminus of P450c17. Conclusion The CYP17 gene of the patient with 17? hydroxylase deficiency is a compound heterozygous mutation. The mutation changes the amino acid sequence of P450c17 enzyme, which in turn affected the enzymatic activity. Arg 96 is essential in P450c17 enzyme activity. Deletion of Asp 487 Ser 488 Phe 489 in exon 8 may be a prevalent mutation causing P450c17 deficiency in Southeast Asia.