In a study of rabbits who were given penicillin, you want to know what dose of penicillin results in a 0.5 probability of curing a rabbit. In this case, the inverse prediction for 0.5 is called the ED50, the effective dose corresponding to a 50% survival rate. Use the crosshair tool to visually approximate an inverse prediction.
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Select Analyze > Fit Y by X.
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Notice that JMP automatically fills in Count for Freq. Count was previously assigned the role of Freq.
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Click OK.
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Place the horizontal crosshair line at about 0.5 on the vertical (Response) probability axis.
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Move the cross-hair intersection to the prediction line, and read the In(dose) value that shows on the horizontal axis.
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In this example, a rabbit with a ln(dose)of approximately -0.9 is equally likely to be cured as it is to die.
Figure 7.78 Example of Crosshair Tool on Logistic Plot
