To calculate IC50, you would need a series of doseresponse data (e.g., drug
concentrations x_{1}, x_{2},
...,x_{n} and growth inhibition y_{1}, y_{2},
...,y_{n}). The values of y are in the range of 01.
Linear Regression
The simplest estimate of IC50 is to plot xy and fit the data with
a straight line (linear regression).
IC50 value is then estimated using the fitted line, i.e.,
Y = a * X + b,
IC50 = (0.5  b)/a.
Log transformation
Frequently, linear regression is not a good fit to doseresponse data.
The responsecurve fits better to a straight line if the xaxis is
logarithmtransformed.
Excel addin
ED50V10 (Readme) is an Excel addin for
calculating IC50/EC50 values. Input your data in the left columns,
and your results will be shown in the right half of the Excel table.
To calculate IC50, input 50 in the "INTERPOLATE..." table (highlighted in blue),
the result will be shown on the right (highlighted in green).
For logtransformation, go to Data Transformation on the upperright, input
3 in the DOSE (Xaxis)/1st cell.
FourParameter Logistic Function
Fourparameter (A, B, C, D) logistic function (or Sigmoidal)
is frequently used to fit doseresponse curves.
The drug data x is in logarithmic form. This is a standard function
in most statistics software.
The parameter C is the estimate of IC50 or EC50.
