Chapter 4

Use these questions to check what you have learned throughout the chapter.

In section 4.1, the book suggests that a historical control may sometimes be favoured over a concurrent control. Can you think of circumstances where this might be true?

A study seeks to explore whether gardeners can influence slug damage to their plants by putting out food to attract hedgehogs. What are the pros and cons of historical and concurrent controls in such a study?

Another study seeks to measure the effect of caffeine on heart rate. Volunteers are recruited on the street outside the teaching hospital where the experiment takes place. They are recruited, walked 200 metres to the room where the study is performed while the purpose of the study is explained to them, they are given a cup of coffee from one of the machines in the cafeteria and connected to a heart rate monitor. How would you recommend the scientist design a control for this experiment? What covariates would it be useful for them to record?

Exam books are often designed so that the name of the student is unavailable to the marker. Discuss the reasoning behind this design in terms of blind procedures.

In section 4.2, the book suggests that between-individual variation is generally higher in field trials than in laboratory studies. Why might this be?

Also in section 4.2, you are cautioned that increasing sample size may not always be an effective method of combating between-individual variation. Explain this caution in your own words.

Describe what is meant by an interaction in a two-factor experiment, and give a biological example of an interaction.

If I wanted to compare sprint speed between students who have either just watched a motivational video or listened to relaxing music, what variables (if any) might I choose to block on?

I have 100 female rabbits, and I want to explore the effects of five different diets on coat condition. I decide to block on age. The rabbits’ ages are known to the nearest month and are approximately evenly spread between 10 and 30 months. What blocks would you recommend and why? What alternatives are there to blocking in this experiment?

You want to compare the attractiveness of peanuts and sunflower seeds to garden birds. Your experiment involved getting householders to provide either peanuts or sunflower seeds to birds in their garden and to record the number and species of birds attending their feeder for 30 minutes each day for 20 days. What covariates might it be useful to record?

If you could only use one of the controls suggested in the hen house experiment in section 4.4.1, would you use silence, Country & Western music or conversation? Defend your choice.

Oxford University Press, Online Resource Centre, Chapter 4.