Oh ok. It showed a linear trend and it was almost parallel to other line hence indicating that the variables were controlled and the heat loss was only adding to inaccuracy of the thing. Hence valid. Although you can argue that the thermometer was not placed in water but rather the beaker but i'd say thats accuracy. and also no heat mats to increase height,but I'd say that doesnt affect validity as much since it was controlled throughout experiment and also that no heat shield was used . However the linear relationship and the parallel shows that valid.
You should say the validity of the experiment is moderate:
Seeing as though the aim was to determine a quantitative relationship between the alkanol used and the heat of combustion, the student effectively achieved this. However, the accuracy of the data is highly compromised to the release of heat from the spirit burner to the surroundings, inability of the beaker and gauze mat to transfer all of the heat to the water (affecting temperature change as it would take longer), as well the the thermometer being placed on the glass and not in the centre of the liquid. But the first sentence is pivotal because it keeps to the aim of the experiment.