Probit method

The Litchfield and Wilcoxon (1949) plotting method was once commonly used. It is certainly a valid method, and it poses no more restrictions on study design than those imposed by the probit method. The Litchfield-Wilcoxon method has become a victim of technology as modem, handheld calculators and the ready availability of simple computer programs have made other methods more convenient to run. 

Inspection of several representative curves indicated that 95% mortality might lie beyond the inflection of the sigmoid curve for data for most of the fractions. However, it seems reasonable to assume that the deposit-mortality relationship may be projected through the 95% level with confidence by the use of the probit method. No advantage... 

Acetone was the primary choice of solvent because of the rapid and spontaneous crystallization characteristics of low boiling solvents (3) from which insecticides may crystallize within a few hours after application. The California five-spined ips [Ips confusus (Le Conte)] was used as the experimental bark beetle for most of the study. The probit method of Finney (16) was used to analyze time-mortality relationships. 

For calculation of the dose-effect curves, the probit method of statistical analysis is used. 

The probit method is perhaps the most widely used method for calculating toxicity vs. concentration or dose. As its name implies, the method used a probit transformation of the data. A probit is a unit of divergence from the mean of a normal distribution equal to one standard deviation. The central value of a probit would be 5.0, representing the median effect of the toxicity test. A disadvantage of the method is that it requires two sets of partial kills. However, a confidence interval is easily calculated and can then be used to compare toxicity results. There are several programs available for the calculation, and as discussed below, they provide comparable results. 

If only one or no partial kills are observed in the data, the Litchfield and Wilcoxin method can be employed. This method can provide confidence intervals but is partially graphical in nature and employs judgment by the investigator. The probit method is generally preferred, but the Litchfield and Wilcoxin method can be used when the partial kill criteria for the probit are not met. 

KTjq Time for 50% knockdown calculated by the probit method ... 

Lethal concentrations were determined with the log-probit method of Finney (75), rather than the procedure of Daum (62) used in earlier investigations (73, 25). [Reprinted with permission from Nickel in the Environment (J. O. Nriagu, ed.). Copyright (c) John Wiley and Sons, Inc., New York.]... 

The recommended method and a more appropriate one is to use the PROBIT method, which can readily handle time-varying situations, including concentration fluctuations (Alp et al., 1990). 

The probit method is a statistical curve fitting method. Furthermore, the results are often extrapolated beyond the experimental data range. This presents a difficult problem for higher doses since the toxicity mechanisms might change. 

The probit method is simply a statistical curve-fitting approach to handle the nonlinear experimental data from exposures. Extrapolation outside the range of the applicable data is unreliable. 

A strength of the probit method is that it provides a probability distribution of consequences and it may be applicable to all types of incidents in CPQRA (fires, explosions, toxic releases). It is generally the preferred method of choice for CPQRA studies. A weakness of this approach is the restricted set of chemicals for which probit coefficients are published. Probit models can be developed from existing literature information and toxicity testing. 

The probit method is only a statistical data fitting technique. The data are also developed based on a constant mean exposure to animals—the approach assumes that the probit equations can be applied to varying concentrations. 

Finally, since the area increases as the square of the distance from the release, the population impact increases in the far field. This increases the sensitivity of the probit method to lower concentrations. 

Lees (1986) summarizes the data from which this relationship was derived. The probit method has found less use for thermal injury than it has for toxic effects. Mathematical models of thermal injuries can be based on a model detailed description of the skin and its heat transfer properties. Experiments have shown that the threshold of pain occurs when the skin temperature at a depth of 0.1 mm... 

Solution From Figure 4.11, the flux levels corresponding to 50% fatalities for 10 and 100 s are 90 and 14 kW/m, respectively. Using the Eisenberg probit method, Eq. (4.7) is rearranged to solve for the thermal radiation intensity I ... 

NOTE a) LD50 values were calculated by Probit method. 

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