new Ext.form.RadioGroup(
{
id : 'Compta'
columns: [150, 150],
vertical: true,
items:
[
{boxLabel: ResCompta.Compta_RemiseCheques ,name: 'Operation' , inputValue: 1}
,{boxLabel: ResCompta.Compta_LettreCheque ,name: 'Operation' , inputValue: 2}
,{boxLabel: ResCompta.Compta_OrdreVirement ,name: 'Operation' , inputValue: 3}
,{boxLabel: ResGlobal.Commun_Autre ,name: 'Operation' , inputValue: 4,checked: true}
]

how can modify the checked box ??