text: If reaction sheet is not assumed to be thin, then there is a chance that one can find a value for  in between zero and unity, although in reality, the reaction sheet is mostly thin compared to turbulent scales. Nevertheless, the general form the density function can be written as
  where  is the probability of finding the progress variable which is undergoing reaction (where transition from zero to unity is effected). Here, we have
  where  is negligible in most regions.