Alpha-level

You see that all 80 patients have their mortality status reported and that just by scanning the frequency tabulation you can tell that more subjects receiving treatment a have died. You also see from the chi-square p- value of 0.0333 that treatment a has significantly more deaths at an alpha level of. 05. 

It is desirable that the number of false positives be small (i.e., there should be a low type I error rate or alpha level). 

Items (2)-(4), which are all to some degree contradictory, require the involved researchers to agree on a set of compromises, starting with the acceptance of a relatively high alpha level (0.10 or more), that is, an increased noise level. 

Type I Error (false positives) Concluding that there is an effect when there really is not an effect. Its probability is the alpha level... 

Duncan s assures a set alpha level or type I error rate for all tests when means are separated by no more than ordered step increases. Preserving this alpha level means that the test is less sensitive than some others, such as the Student-Newman-Keuls. The test is inherently conservative and not resistant or robust. 

Cuchacovich, M., Ferreira, L., Aliste, M., et al. (2004) Tumour necrosis factor-alpha (TNF-alpha) levels and influence of -308 TNF-alpha promoter polymorphism on the responsiveness to infliximab in patients with rheumatoid arthritis. Scandinavian Journal of Rheumatology. 33, 228-232. 

Power calculations for survival data are more complex due to the nature of the analyses as well as factors that are involved in the accrual of participants (i.e., follow-up time, prevalence of risk factor, etc.). The following example is based on the method discussed by Simon and Altman (41) using an 18-month overall survival rate of 40%, two-sided alpha level of 5%, and no attrition for varying levels of risk factor prevalence and hazard ratios. 

D4. Davas, E. M., Tsirogianni, A., Kappou, I., Karamitsos, D., Economidou, I., and Dantis, P. C., Serum IL-6, TNF alpha, p55 srTNF alpha, p75 srTNF alpha, srIL-12 alpha levels and disease activity in systemic lupus erythematosus. Clin. Rheumatol. 18, 17—22 (1999). 

Rat (Fischer- 344) 2-4 wk 5 d/wk 5 hr/d 0.36 (decreased tumor necrosis factor-alpha levels and production of superoxide anion and hydrogen peroxide and increased nitric oxide production) Cohen et al. 1998 BaCr04 (VI)... 

Raju, J. and Bird, R.P. (2006) Alleviation of hepatic steatosis accompanied by modulation of plasma and liver TNF-alpha levels by Trigonella foenum-graecum (fenugreek) seeds in Zucker obese (fa/fa) rats. International Journal of Obesity 30(8), 1298-1307. 

Tuglu C, Kara SH, Caliyurt O, Vardar E, Abay E (2003) hicreased serum tumor necrosis factor-alpha levels and tieatment response m major depressive disorder. Psychopharmacology (Berl) 170 429 33. 

Crespo, J., Rivero, M., Fabrega, E., Cayon, A., Amado, J.A., Garcia-Unzeta, M.T., Pons-Romero, F. Plasma leptin and TNF-alpha levels in chronic hepatitis C patients and their relationship to hepatic fibrosis. Dig. Dis. Sci. 2002 47 1604-1610... 

Mogi M, Harada M, Narabayashi H, Inagaki H, Minami M, Nagatsu T (1996) Interleukin (IL)-l beta, IL-2, IL-4, IL-6 and transforming growth factor-alpha levels are elevated in ventricular cerebrospinal fluid in juvenile parkinsonism and Parkinson s disease. Neurosci Lett 211 13-16. 

Hemmer CJ, Hort G, Chiwakata CB, Seitz R, Egbring R, Gaus W, Hogel J, Hassemer M, Nawroth PP, Kern P, Dietrich M. Supportive pentoxifylline in falciparum malaria no effect on tumor necrosis factor alpha levels or clinical outcome a prospective, randomized, placebo-controlled study. Am J Trop Med Hyg 1997 56(4) 397 03. 

Err on the side of including more factors than less. Use generous inclusion criteria. For example, consider the use of alpha levels of from 0.1 to 0.2 for testing whether to keep parameters in the model. 

Mussi A, Bonifati C, Carducci M, et al. Serum TNF-alpha levels correlate with disease severity and are reduced by effective therapy in plaque-type psoriasis. J Biol Regul Homeost Agents 1997 11 115-118. 

Kalehua, A. N., Taub, D. D., Baskar, P. V., Hengemihle, J., Munoz, J., Trambadia, M., Speer, D. L., De Simoni, M. G., and Ingram, D. K. Aged mice exhibit greater mortality concomitant to increased brain and plasma TNF-alpha levels following intracerebroven-tricular injection of lipopolysaccharide. Gerontology 46 (2000) 115-128. 

For a given sample size n, alpha and beta errors are inversely related in that, as one reduces the a error rate, one increases the j3 error rate, and vice versa. If one wishes to reduce the possibility of both types of errors, one must increase n. In many medical and pharmaceutical experiments, the alpha level is set by convention at 0.05 and beta at 0.20 (Sokal and Rohlf, 1994 Riffenburg, 2006). The power of a statistic (1 — /3) is its ability to reject both false alternative and null hypotheses that is, to make correct decisions. 

Gopaul, N. K., Anggard, E. E., Mallet, A. I., Betteridge, D. J., Wolff, S. P., and Nourooz-Zadeh, J., Plasma 8-epi-PGF2-alpha levels are elevated in individuals with non-insulin dependent diabetes mellitus. FEBS Lett. 368, 225-229 (1995). 

The alpha level or Type I error rate-. By convention, the level of alpha is usually set at. 05, but this figure is arbitrary. The Type I error rate is the probability of falsely rejecting the null hypothesis that is, the chance of concluding that a systematic relationship exists in the population when it, in fact, does not. This possibility tends to predominate the consciousness of investigators and many post-hoc techniques (e.g., the Bonferoni inequality, the Newman-Keuls test, etc.) have been developed to control it. 

For experiments 1—3, in which individual males were simultaneously presented with two models in a choice test, paired, within-subjects chi-square tests were used to compare the frequency of males that clasped female-scented vs. unscented models (Exp. s 1,2) or female-scented vs. male-scented models (Exp. 3 see Snedecor Cochran, 1989). Wil-coxon signed-ranks tests were used to compare the number of clasps per male of scented vs. unscented models. In experiments 4 and 5, chi-square tests were used to compare the frequency of males that clasped models in the different treatment groups. Mann-Whitney U-tests were used to compare the number of clasps per male in the different groups. An alpha level of p <. 05, one-tailed was required for statistical significance. 

Fluorescence induction was measured at 72 uE m s Photochemical (qQ) and non-photochemical (qN) fluorescence quenching were measured with 250 ms pulses (2000 uE m s ) and calculated as described in (8). Two sample-t tests were conducted at the. 05 and. 10 alpha levels and their results are reported as p-values. 

To perform a t-test on a correlation coefficient, one must determine the probability of being correct when concluding that the correlation coefficient is significant. This degree of chance or probability is referred to as the alpha level. A commonly used... 

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