The two-period crossover design with baseline measurements

Significant variation in salivation by short-term suggestive intervention: a randomized controlled cross-over clinical study

INTRODUCTION

Most dental procedures require a dry working environment. Although many evaporative drying methods are available, an additional reduction of salivary flow would often be helpful.

METHODS

This prospective randomized cross-over study compares salivary production in 31 volunteers during direct, indirect, and non-suggestive (control group) intervention. Overall, each volunteer underwent four salivation measurements, i.e. two measurements during two different types of hypnotic suggestion (indirect and direct) arranged in random order and two control sections. All four measurements were conducted successively.

RESULTS

Both suggestive methods significantly reduced salivary production in comparison to the two control sections (direct suggestion Δ = 1.46 grams per 5 min, p < 0.001, indirect suggestion Δ = 0.94 grams per 5 min, p = 0.039). Direct suggestion showed a significantly higher reduction of salivary production than indirect suggestion (Δ = -0.53 grams per 5 min, p = 0.001).

CONCLUSIONS

Hypnotic suggestion represents a simple and inexpensive method to reduce salivation and could thus create a better working environment for more comfortable dental treatments for both patients and dentists.

Regulatory affairs in biotechnology: optimal statistical designs for biomedical experiments

One of the major issues in all applications of biotechnology is how to regulate the process through which new technological information is produced. The end products of biotechnological applications are diverse (e.g., better drugs, better interventions, better fertilizers). Such applications should be properly regulated to obtain valid scientific findings in the most efficient way possible. Some statistically optimal designs are more popularly employed than others as regulatory tools in medical, pharmaceutical and clinical trials. The statistical and practical properties (strengths and weaknesses) are presented to better appreciate their optimality. Recent developments on some related issues are also reviewed.

Analysis of the 2 x 2 crossover design with subsampling

Subsampling can be used in experimental design to investigate extraneous sources of variability. One useful strategy is to make independent replicate measurements of the response variate. This can be achieved in clinical studies, for example, by dividing a blood or urine specimen from each of a sample of subjects into aliquots and processing these through a chemistry laboratory in such a way that the replicate determinations are independent. This procedure can be used to increase design efficiency. This paper addresses considerations in the design and analysis of the 2 x 2 crossover plan with this type of subsampling.

Regression toward the mean in 2 x 2 crossover designs with baseline measurements

Clinical trials often involve the study of patients whose values for some quantitative characteristic are in the extreme of the distribution for the normal population. Subsequent observation of the characteristic of interest will ordinarily tend to regress toward the mean of the distribution even in the absence of a treatment effect. We consider the impact of regression toward the mean in the analysis of data arising in 2 x 2 crossover trials with two baseline measurements. The usual summary statistics can be especially misleading in the presence of regression toward the mean.

A critique of recent research on the two-treatment crossover design

A critical review is provided of recent articles on two-treatment crossover trials that purport to provide solutions to the problems caused by the existence of differential carryover effects. Included in the review are papers suggesting that extra-period designs be employed in which treatments follow themselves, that the bias caused by a difference between the carryover effects may sometimes be ignored, that changes from baseline be analyzed, and that a series of preliminary tests using baseline values be performed. The validity of many of these suggestions is shown to depend on whether mathematical assumptions that are generally unverifiable are satisfied. The two-period crossover design is recommended only when prior experience suggests that its assumptions are likely to be satisfied.