(a) (i) What is the purpose of a hypothesis in the scientific method? (ii) Explain what is meant by double-blind testing in scientific experimentation - Leaving Cert Biology - Question 9 - 2015

In double-blind testing, neither the participants nor the experimenters know who belongs to the control group and who belongs to the experimental group. This method prevents bias in the results and ensures that any observed effects can be attributed to the treatment being tested, rather than to external influences or expectations.

It is important for all the tadpoles to come from the same mother because this ensures that they are genetically similar. This genetic homogeneity helps control for variations in growth rates that could arise from genetic differences, allowing for a more accurate assessment of the effect of iodine concentration.

A control is essential in scientific experiments as it provides a standard or baseline against which the experimental group can be compared. This allows researchers to isolate the effect of the variable being tested, thereby increasing the validity of the results.

Using 25 tadpoles in each group allows for a larger sample size, which makes the results more statistically significant. It also helps in accounting for potential illness or mortality among the tadpoles, ensuring enough individuals are present to yield reliable data.

The investigation would have been carried out by assigning different concentrations of iodine solution to each group of 25 tadpoles. The experimenter would use water from the pond and measure changes in growth over a set period, documenting the results for each concentration, with one group receiving no iodine as the control.

Two factors that the scientist would have kept constant include the temperature of the water and the duration of exposure to the iodine solution. Additionally, the amount of food provided and the volume of water in each tank could also be controlled to ensure a fair comparison across all groups.