Circumferential adjustment of ultrasound probe position to determine the optimal approach to the internal jugular vein: a noninvasive geometric study in adults.

Circumferential adjustment of the position of a two-dimensional ultrasound (US) probe around the neck has been recommended as a strategy for reducing the potential for unintentional common carotid artery puncture during internal jugular venous (IJV) cannulation. We obtained multiple high-resolution US images bilaterally from the necks of 107 adult subjects and analyzed these to determine the degree to which this strategy permits identification of a pathway from the skin to the IJV that minimizes venoarterial overlap while maximizing venous target (angular) width. The method consistently permitted identification of an approach to the IJV superior to that obtainable with any one of four popular surface anatomy-based ("blind") approaches and was even more powerful if used in concert with a US-guided 1) adjustment of the degree of head rotation, 2) choice between a high and low approach, and 3) choice between the right and left IJV. Use of a high-resolution US imaging device also permitted identification of the precise boundaries of additional cervical anatomic structures (nontarget vessels, lymph nodes, and the thyroid gland) potentially relevant to selection of an optimal approach to the IJV.

On confidence intervals for within-subjects designs.

Confidence intervals (CIs) for means are frequently advocated as alternatives to null hypothesis significance testing (NHST), for which a common theme in the debate is that conclusions from CIs and NHST should be mutually consistent. The authors examined a class of CIs for which the conclusions are said to be inconsistent with NHST in within-subjects designs and a class for which the conclusions are said to be consistent. The difference between them is a difference in models. In particular, the main issue is that the class for which the conclusions are said to be consistent derives from fixed-effects models with subjects fixed, not mixed models with subjects random. Offered is mixed model methodology that has been popularized in the statistical literature and statistical software procedures. Generalizations to different classes of within-subjects designs are explored, and comments on the future direction of the debate on NHST are offered.

The difference between t and z and the difference it makes.

Tests of the null hypothesis for comparisons involving sample means use the t test when the conditions of the z test cannot be met. The 2 tests have different rationales and can lead to different conclusions regarding significance. In the present study, the authors compared the properties of t and z in simulation runs. The differences in the results are a result of fluctuations in the t test sample variances that do not exist in the z test, and those differences lead to differences in designating the significance of comparisons.

Functional anatomy of the null hypothesis and of tests of it.

The author compared simulations of the "true" null hypothesis (zeta) test, in which sigma was known and fixed, with the t test, in which s, an estimate of sigma, was calculated from the sample because the t test was used to emulate the "true" test. The true null hypothesis test bears exclusively on calculating the probability that a sample distance (mean) is larger than a specified value. The results showed that the value of t was sensitive to sampling fluctuations in both distance and standard error. Large values of t reflect small standard errors when n is small. The value of t achieves sensitivity primarily to distance only when the sample sizes are large. One cannot make a definitive statement about the probability or "significance" of a distance solely on the basis of the value of t.