Drawbacks of Principal component analysis
  One of the main tasks for present and future dark energy surveys is to
determine whether the dark energy is dynamical or not. To illustrate this from
data, it is commonly used to parameterize the dark energy equation of state w
as several piecewise constant wis using the principal component analysis (PCA)
method over finite redshift bins. We show that there is only $j-1$ free
parameters wis if we choose the redshift as j bins. Without this constrain, one
obtains the inconsistent results from the data analysis. Furthermore, if w
decreases with non-negligible ratio as z does, then PCA fails to reproduce the
original behavior of w. Also, time varying w can be confused with the incorrect
value of constant one when the decreasing (or increasing) ratio of w is small
but not negligible.
