HSA 2009 Biology Item 52

Use the information and the chart below to answer the following item.

Several students are conducting an experiment to test the effect of exercise on heart rate. Students do 20 sit-ups in one minute of exercise. The data chart below shows the heart rate at one-minute intervals for each student.

The students decide to ignore any results that were too far above or below their prediction. They repeat these trials until a value closer to their prediction is obtained. The students’ decision to ignore data and repeat trials affects the experiment by

1. introducing bias
2. decreasing controls
3. increasing accuracy
4. eliminating variables