Question: What happens to the temperature of a cup of hot coffee when left out in the winter compared to the same cup left out in the summer?
You all know from everyday experience that, in Winter, the rate of cooling (how quickly the fall in temperature happens) and the rate of heat transfer from the coffee to its surroundings are both greater/higher than in Summer.
The only real difference between these two scenarios is the size of the temperature difference between the hot coffee and the cooler surroundings; everything else has been kept constant to allow us to see the effect more clearly.
Therefore the rate of cooling must depend on the size of the temperature difference since all the other variables have been kept constant.
So we have also made a fair test, because only one independent variable (the outside temperature) was free to change.