# Dose response parameters

This page discusses metrics derived from curve fits, like IC_{50}s. For details on the curve fitting process
and differences between viability and DIP rate dose response curves, see curve fitting.

## IC_{50} and inhibitory concentrations

Image: Definition of IC_{50} shown on a dose response curve

Half-maximal inhibitory concentration (IC_{50}) is a measure of drug potency expressed in molar units (M).
It specifies the concentration at which the dose response curve reaches 0.5 on the y-axis (for DIP rate dose response
curves, this is 0.5 on a relative scale y-axis). The IC_{50} may be undefined in the event that the maximum
effect on the fitted curve does not reach 0.5 on the y-axis. On some plots in Thunor, this will show as `NA`

.

Other inhibitory concentrations are defined analogously for any response value between 0 and 1. For DIP-based dose
response curves, which can go below zero on the y-axis, IC_{100} denotes the concentration at which the dose
response curve crosses the y-axis.

## EC_{50}, E_{50}, and effective concentrations

Image: Definition of EC_{50} shown on a dose response curve

Half-maximal effective concentration (EC_{50}) is a measure of drug potency expressed in molar units (M).
It specifies the concentration at which the dose response curve reaches 50% of the maximum effect observed, based on the
fitted model - i.e. the halfway point between the curves maximum and minimum y-values. The response/y-value of the curve
at this point is the half-maximal effect (E_{50}).

### Relative vs absolute values

For DIP rate curve fits, EC and E values can be measured on a relative DIP rate axis, or an absolute one. Thus, metrics with "relative" in their name are measured on the relative axis. Viability is already a relative metric, so this distinction does not apply (all values correspond to viability relative to control).

## E_{max}

Image: Definition of E_{max} shown on a dose response curve

The maximal effect (E_{max}) denotes the fitted curve's effect (i.e. the y-value) at the maximum observed
concentration (the highest drug concentration for which there is data).

## E_{max} (observed)

Image: Definition of E_{max} observed shown on a dose response curve

The observed maximal effect (E_{max} (observed)) is the maximum effect observed experimentally. Note that this
is not necessarily observed at the maximum concentration.

## Activity Area

Image: Definition of Activity Area shown on a dose response curve

Activity area is the area above the dose response curve, up to the no response value (1 on the y-axis), bounded by the minimum and maximum observed concentrations.

## Activity area (observed)

Image: Definition of Activity Area (observed) shown on a dose response curve

Activity area (observed) is the area above the observed response values, up to the no response value (1 on the y-axis).
Between doses, a straight line extrapolation is used (on a log_{10} dose x-axis). Response values above 1 are
truncated at 1. If there are multiple response values for a particular concentration (replicates), the mean average is
used.

## Hill coefficient

Image: Definition of Hill coefficient shown on a dose response curve

The Hill coefficient quantifies the rate of onset of a drug's effect as drug concentration increases. It affects the steepness of the dose response curve.