A scientist is researching the effect of an energy drink on a group of twenty men. State two characteristics of the sample which may influence the results.

So this question is looking at how characteristics of different people may affect how they respond to an energy drink. A good way to start thinking about this question is to think about what makes the people in the sample different to each other, as most of this may have an influence.

So we know that the sample are all the same sex (male) but we have no information on things such as;

-Age (older people may respond differently to younger)

-Whether they are healthy/have existing medical problems (some health problems may affect how the particpant responds)

-BMI 

-If they regulary consume energy drinks (or even caffinated drinks)

-Lifestyle (such as if they regulary exercise/smoke/drink alcohol)

As you can see, something as simple as a particpants age can have an influence on the results of the research. This is important as it may produce invalid results or even may demonstrate an important trend (e.g. Older men gain a lot more energy from the drinks so should drink less).