Facial tumours in Tasmanian devils

Topic: Studies, data, and evidence · more questions on this topic
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Thirty devils with facial tumours are split into two groups of 15. One group receives a new medicine and the other is untreated. Tumour size is recorded at the start and after 12 weeks.

  1. Is this an observational study or a designed experiment? What is missing from the description before the quality of the comparison can be judged?

  2. Which variable is the response, and what type of variable is it?

  3. Which variable is explanatory, and what type of variable is it?

  1. It is a designed experiment: the investigators decided which devils got the medicine. Before judging it we would want to know whether the groups were formed at random, whether whoever measured the tumours knew which devils were treated, whether the untreated group received a placebo or at least the same handling, and how comparable the devils were at the start in age, sex and tumour size. Fifteen per group is small, so a modest effect could be missed. Tumour size alone may also miss what matters, such as survival.

  2. The response is the change in tumour size over the 12 weeks, or the size at 12 weeks adjusted for the starting size. It is quantitative and continuous. The question is whether the medicine changes it.

  3. The explanatory variable is the treatment, medicine or none. It is categorical, a factor with two levels, and it is set by the investigators.