Question

A farmer wishes to test the effects of a new fertilizer on her tomato yield. She has four equal-sized plots of land-- one with sandy soil, one with rocky soil, one with clay-rich soil, and one with average soil. She divides each of the four plots into three equal-sized portions and randomly labels them A, B, and C. The four A portions of land are treated with her old fertilizer. The four B portions are treated with the new fertilizer, and the four Cʹs are treated with no fertilizer. At harvest time, the tomato yield is recorded for each section of land.

1. Is this an observational study or an experiment?

2. Name at least 2 factors in this study.

Answer 1

a) An observational study is the one where there is no manipulation of the setting. The study involves observation of certain parameter(s) on groups without any intervention. These groups are pre-existing and thus such studies are done in a more natural setting. However, the existing biases in the sample cannot be eliminated in such studies.

An experimental study is the one where the setting is manipulated. The groups are assigned by the experimenter in a randomized manner. Thus, the setting is no longer natural and hence is often difficult to be extrapolated to real-life scenarios. However, this allows to account for biases.

Here, the fertilizer was given to parts of the plots. Thus, the study wasn't observational. It was done in a controlled setting and hence is an experimental study.

b) Factors refers to the parameters that the study is based on. Here, the parameters of interest are the type of the soil which can be sandy soil, rocky soil, clay-rich soil or average soil, and type of fertilizer used which can be old, new or no fertilizer. Hence, the factors of the study are soil type and fertilizer type.