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.

Repeating the experiment, or running multiple individuals per condition, accounts for biological variation and measurement error. NESA expects students to identify a minimum number of replicates (often 5 to 10 per condition) and to repeat the experiment at least three times.

In human or animal studies, randomisation assigns subjects to treatment groups by chance to reduce selection bias. Blinding prevents the researcher or the subject from knowing the treatment assignment. Double-blind designs are the gold standard in clinical trials.

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).

Examiners reward thorough lists. Three to five is the standard.

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

With two simultaneous changes you cannot attribute the result to either variable. :::