{'$match':
      {'charRange':
          {'$gt': 0},
      'countPar':
          {'$gte': 0}
},

{'$group':
      {'_id':
          {'charRange': '$charRange', 'parNr': '$countPar'},
       'count':
          {'$sum': 1}
      }
},

{'$project':
     {'optimalCredit':
          {'$cond':
              {'if':
                    {'$gt': [
                             {'$avg': 'credit'},
                             {'$stdDevPop': 'credit'}
                            ]
                    },
               'then':
                    {'$avg': '$credit'},
               'else':
                    {'$stdDevPop': '$credit'}
              }
          }
     }
},

{'$sort':
     {'optimalCredit': -1}
}

{'$match':
     {'charRange':
          {'$gt': 0},
     'countPar':
          {'$gte': 0},
     }
},

{'$group':
    {'_id':
          {'charRange': '$charRange', 'parNr': '$countPar'},
           'creditAvg':
                 {'$avg': '$credit'},
           'creditStdDev':
                 {'$stdDevPop':'$credit'},
           'count': {'$sum': 1}
    }
}

{'_id': {'parNr': 7, 'charRange': 1500}, 'optimalCredit': None}

{'_id': {'charRange': 1500, 'parNr': 7}, 'creditStdDev': 19.430788855719562, 'count': 9, 'creditAvg': 16.0}

{'_id': {'parNr': 7, 'charRange': 1500}, 'count': 9, 'optimalCredit': 19.430788855719562}