Statistics control groups

There should be sufficientiy large numbers of animals to allow a quantitative determination of the average response and the range of responses, including the demonstration of hypersensitive populations. When objective procedures are undertaken, these should be sufficient to allow vaUd statistical comparison to be made between treated and control groups. 

Lowcst-Obsciwcd-Advcrsc-El fcct-Lcvel (LOAEL) In dose-response experiments, the lowest exposure level at which there are statistically or biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control group. 

We might also note here, almost parenthetically, that if the hypothesis test gives a statistically significant result, it would be valid to calculate the sensitivity of the result to the difference between the two groups (i.e., divide the difference in the means of the two groups by the difference in the values of the variable that correspond to the experimental and control groups). 

Growth inhibited organ/BW ratios increased in brain, liver, spleen, kidney, heart (females), and testes. Mammary and pituitary tumors commonly observed in treated and in control groups. No statistically significant difference in number or type of neoplasms, except for mammary tumors... 

The use of one double-sized group is sometimes preferred because it may provide a better estimate of the true control tumor incidence than that provided by a smaller group. Nevertheless, more statistical power would be obtained by assigning the additional animals equally to all dose groups rather than to the control group only, if power is a primary consideration (Weaver and Brunden, 1998). 

Irrespective of the specific protocols used, all carcinogenicity studies end with a statistical comparison of tumor proportions between treated and control groups. This analysis is necessary because the control incidence of most tumor types is rarely zero. In the unlikely case that a type of tumor is found in treated animals but not in concurrent or appropriate historical controls, it is reasonable to conclude that the tumor is drug-related without statistical analysis. 

The first precise or calculable aspect of experimental design encountered is determining sufficient test and control group sizes to allow one to have an adequate level of confidence in the results of a study (that is, in the ability of the study design with the statistical tests used to detect a true difference, or effect, when it is present). The statistical test contributes a level of power to such a detection. Remember that the power of a statistical test is the probability that a test results in rejection of a hypothesis, H0 say, when some other hypothesis, H, say, is valid. This is termed the power of the test with respect to the (alternative) hypothesis H. ... 

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