PHP array checkbox and radio default value

I have the code below in my wordpress options page. I can’t define the default values for checkbox and radio.

array(
    "name" => "Post Thumbnails",
    "desc" => "Choose if you want to display <strong>post thumbnails</strong> or not.",
    "id" => $shortname."_post_thumbs",
    "type" => "checkbox",
    "std" => "checked"      
),  

array(
        "name" => "Example",
        "desc" => " The Descriptions",
        "id" => $shortname."_case_thumb",
        "type" => "radio",
        "options" => array("nothumb" => " Display nothing", "defthumb" => " Display thumbnail"),
        "std" => "nothumb"      
    ),

For the checkbox tried also "std" => "true" and "std" => " "… but didn’t work. Thanks!