How to use a movieclip path from a variable?

so.. i have a variable that will change,

and i want to make it my movieclip path, example

var address1:String = New String

if ( someThing > anotherThing) {

     address1 = "mc_big.mc_test"

}

else {

    address1 = "mc_small.mc_test"

}

address1.gotoAndPlay ("this is a test")

well.. that dosent work... i need a way to do that..  im hours trying to figure it out, and nothing..

the only result i got, only works with a short address..

exemple

address1 = "mc_big"

this[address1].gotoAndPlay ('this works!!")

address1 = "mc_big.mc_test"

this[address1].gotoAndPlay ('this dont workt")

any help?