What are $Omh^2(z_1,z_2)$ and $Om(z_1,z_2)$ diagnostics telling us in
  light of $H(z)$ data?
  Two-point diagnostics $Om(z_i,z_j)$ and $Omh^2(z_i,z_j)$ have been introduced
as an interesting tool for testing the validity of the $\Lambda$CDM model.
Quite recently, Sahni, Shafieloo $\&$ Starobinsky (2014) combined two
independent measurements of $H(z)$ from BAO data with the value of the Hubble
constant $H_0$, and used the second of these diagnostics to test the
$\Lambda$CDM model. Their result indicated a considerable tension between
observations and predictions of the $\Lambda$CDM model. Since reliable data
concerning expansion rates of the Universe at different redshifts $H(z)$ are
crucial for the successful application of this method, we investigate both
two-point diagnostics on the most comprehensive set of $N=36$ measurements of
$H(z)$ coming from the BAO and differential ages (DA) of passively evolving
galaxies. We discuss the uncertainties of two-point diagnostics and find that
they are strongly non-Gaussian and follow the patterns deeply rooted in their
very construction. Therefore we propose that non-parametric median statistics
is the most appropriate way of treating this problem. Our results support the
claims that $\Lambda$CDM is in tension with $H(z)$ data according to the
two-point diagnostics developed by Shafieloo, Sahni and Starobinsky. However,
other alternatives to the $\Lambda$CDM, such as wCDM or CPL models perform even
worse. We also notice that there are serious systematic differences between BAO
and DA methods which ought to be better understood before $H(z)$ measurements
can become competitive to the other probes.
