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and
where p is the probability of response at dose x, for the reference standard and the test sample respectively.
φ is the cumulative distribution function for the standard normal distribution.
The ED50* is the dose where the probability of response is 0.5; log(ED50) = -A/B.
* Also known as midpoint, or EC50
 is the slope of the relationship between p and log (x)at the ED50.
A is the intercept and B is commonly referred to as the slope coefficient.
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For the parallel fit, B is the same for the two samples:
and 
