Quick questions on Variables and experimental design: HSC Investigating Science Module 5

The variable that the researcher deliberately changes to test its effect. There is normally one independent variable per experiment, set at multiple levels (e.g. 0, 10, 20, 30 degrees Celsius).

The variable that the researcher measures to see how it responds. It depends on the independent variable.

The variables held constant across all treatment groups to prevent them from confounding the result. In the fertiliser experiment, controlled variables include soil type, watering schedule, sunlight, plant variety and starting plant size.

A group treated identically to the experimental groups except that it receives no treatment (or a placebo). The control group establishes the baseline against which treatment effects are compared.

Comparative design. Two or more groups treated differently and compared. Most school experiments fit here.

Two or more groups treated differently and compared. Most school experiments fit here.

One subject measured repeatedly over time as the independent variable changes.

Multiple independent variables changed simultaneously to detect interactions (e.g. temperature and pH on enzyme activity).

A single measurement per condition cannot account for variability. Always specify replicates.

A student investigates whether "music helps memory." Identify the independent and dependent variables they should specify, and list three controlled variables required for a valid investigation. [4 marks]

A trial compared two batches of CSIRO drought-tolerant wheat at sites in Wagga Wagga and Dubbo. Yields were higher in Dubbo. Explain why this difference does not establish that the wheat variety performs better in Dubbo, and design a more valid comparison.

A pharmaceutical company tests a new analgesic on 200 volunteers. (a) Explain the difference between single-blind and double-blind designs. (b) Explain why double-blind is preferred.