Assignment 1

Designing the experiment: selection of variables

In order to design an experiment several things must be taken into consideration.

Variables:

dependant and indipendant?

continuous or discreet?

Continuous variables are variables in which a wide range of values are expected or can be obtained. for example :age, time, height, weight, % of polymerization and light wavelengths. Usually continous variables are quantitative.

Discreet (discontinuous) variables are variables in which take a qualitative property. For example: colour, sex and species are discreet. It is possible to convert discreet variables into continous variables. For example instead of evaluating based on colour, it can be converted to 'reflected light wavelength"

Examples of variables seen in assignment 1 are:

temperature

leaf disc size

light intensity

level of destarched leaves

plant species

sugar type

sugar quantity

Bear in mind to include variables that have a possibility in affecting the experiment (either as a source of error or as a contributor to results)

things like leaf thickness, no. of stomata per disc are also variables but since they cannot be controlled are ignored. They can be used however to explain possible discrepancies in your results.

In an ideal experiment only one indipendant variable is mantained and its effect on a dependant variable is monitored. All other variables are mantained or kept constant.

If for exampler we want to examine the effect of sugar concentration then the following things must be mentioned.

1. Stock sugar solution was diluted to the required concentrations. for example to create a 5g/L solution from a 10 g/L solution, take 100ml stock and add equal volume of dH2O.
2. State all your required sugar concentration. Make sure you select a minimum of 5 ranges if the question does not specify no. and make sure they are reasonable values. for example do not say you are going to select a 12g/L solution when your stock is only 10g/L
3. Mention which variables you will keep constant. for example :

• all leaf discs were cut with a cork borer with a 1cm diameter
• apparatus was set up in a regulated water bath with an electronic thermocoupler
• apparatus was set up under a light source with constant light intensity (avoid saying under the sun as sunlight intensity can shift)
• mention no. of leaf discs per experiment eg. 30 or 100 (select values which are reasonable for the experiment) if large samples are readily available then use large samples
• time of experiment. how long does the experiment need to run. for something like this results could change very slowly over time so state something like 2 to 4 hrs.

note some methods automatically become precautions, especially if you state that the same step is conducted across the experiment (eg. mantain light intensity for all experiments, mantain temperature for all experiments.
next : designing a control