= PDAFomi_diag_stats = 

This page documents the routine `PDAFomi_diag_stats` of PDAF, which was introduced with PDAF V3.0. This is part of the [wiki:PDAFomi_observation_diagnostics PDAF-OMI observation diagnostics module].

The routine returns a pointer to an array of a selection of 6 statistics comparing the observations and the observed ensemble mean for each observation type. The statistics can, for example, be used to plot a Taylor diagram.

Usually all PDAFomi_diag routines are called in `prepoststep_pdaf` where the observation information can be retrieved and analyzed. 

The interface is: 
{{{
  SUBROUTINE PDAFomi_diag_stats(nobs, obsstats_ptr, verbose)

    INTEGER, INTENT(inout) :: nobs                     ! Number of observation types
    REAL, POINTER, INTENT(inout) :: obsstats_ptr(:,:)  ! Array of observation statistics
          ! Included statistics are:
          !  (1,:) correlations between observation and observed ensemble mean (corr)
          !  (2,:) centered RMS difference between observation and observed ensemble mean (cRMSD)
          !  (3,:) mean bias (observation minus observed ensemble mean, 'bias y-Hx')
          !  (4,:) mean absolute difference between observation and observed ensemble mean (MAD)
          !  (5,:) variance of observations (STDDEV(y))
          !  (6,:) variance of observed ensemble mean (STDDEV(Hx))
    INTEGER, INTENT(in) :: verbose                     ! Verbosity flag, >0 to write screen output
}}}

For verbose>0 a list of statistics is printed for each observation type in the form
{{{
PDAFomi   obs-ID   corr      cRMSD     bias y-Hx      MAD      STDDEV(y)   STDDEV(Hx)
PDAFomi      1     0.611   7.084E-01   4.376E-02   5.774E-01   8.787E-01   7.179E-01
}}}

**Note:**
 * The computed statistics are for the global model domain. Thus, in case of a parallelized model, all process sub-domains are taken into account and calling `PDAFomi_diag_stats` will return the same value for all processes.
 * In Fortran user code the pointer should be declared in the form[[BR]] `REAL, POINTER :: obsstats_ptr(:)`[[BR]] It does not need to be allocated. The target array has the size `(6, nobs)`.
 * If the observation diagnostics have not be activated by using [wiki:PDAFomi_set_obs_diag] the pointer array will not be set and `nobs=0` is. One can check this value before assessing the pointer array
 * The routine returns the centered RMSD as displayed in Taylor diagrams. The non-centered RMSD can be computed using [wiki:PDAFomi_diag_obs_rmsd].