In the study by Pepperberg (parrot learning), many variables were controlled.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

1. because it ensures that the variable having influence is the IV;
2. so it improves validity; (dependent on first marking point)
3. because it helps to standardise the experience of participants;
4. so it improves reliability; (dependent on first marking point)
5. so the experimenter can make a judgment about causality;
6. What he did during the day was not controlled (e.g. he could choose to go to the gym or not);
7. This could have affected the results because he would have been exposed to different colours / shapes in different places;
8. What he ate was not controlled (e.g. he had a range of foods such as different nuts and vegetables);
9. These are different colours so he would have been exposed to some recently;
10. What toys he played with were not controlled (because he could ask for different ones);
11. These were different shapes/materials/colours so he would have been exposed to some types recently;