The main value of a checkbox is a boolean, indicating whether or not the checkbox is checked. The following values will check the checkbox:
  • true
  • 'true'
  • '1'
  • 'on'
Above doesn't seem to work

Code:
{
                      id        : 'complexity',
                      name      : 'complexity',
                      xtype:"checkbox",
                      value: "1"
 }