The independent variable is the type of circuit and the dependent variable is the light. If the Christmas lights are arranged in a parallel circuit then when 1 bulb burst the other bulbs will light up.
I noticed that all lights were lit up and bright.
I noticed when one bulb burst (removed) the rest of the lights stayed on and bright.
If the Christmas lights are arranged in a series circuit then when 1 bulb bursts (is removed) the other bulbs will not light up.
I noticed that all the lights were on but dim.
I noticed that all the lights turned off when one was removed.
A parallel series is the best series to use for Christmas lights because when 1bulb goes out the rest stay on.