Unit 3 Investigative Biology

SQA

Success Criteria  Explain the need for a pilot study in terms of the dependent variable, the independent variable, confounding variables and replicates.

Pilot studies  Integral to the development of any investigation, a pilot study is used to help plan procedures, assess validity and check techniques.  This allows evaluation and modification of experimental design.  A pilot study can be used to develop a new protocol or to enable an investigator to become proficient in using an established protocol.

Pilot study example  Daphnia magma is a fresh water crustacean often used in laboratory studies of physiology or toxicity.  It is readily available and often used to measure the effect of drugs such as caffeine and ethanol on heart rate. Heart

Pilot studies  How could we set up a pilot study to see if Daphnia could provide a suitable laboratory model to look at the effect of ethanol on heart rate?  The aim here is to raise questions about the issues that may need to be resolved.  Planning questions: Can stocks be sourced and maintained in a school laboratory? How can we measure the dependent variable, heart rate? How can we alter and set ranges for the independent variable, ethanol concentration?

Pilot studies Can stocks be sourced and maintained in a school laboratory? Yes, eg Blades Biological How can we measure the dependent variable? Heart rate can be measured using a microscope but the way this is done will require careful consideration. Try it yourself with this link: ch?v=MJCnZ0pB3q4

Confounding Variables??!  Due to the complexities of biological systems, other variables besides the independent variable may affect the dependent variable.  A confounding variable is therefore an extraneous (“outside” ) variable that is linked to both the independent and dependant variable.  These confounding variables must be held constant if possible, or at least monitored so that their effect on the results can be accounted for in the analysis.

What could they be?  Daphnia are very mobile therefore how can they be kept in the field of view?  Does manipulating Daphnia onto a slide for viewing cause an increase in heart rate?  Does prolonged exposure to the heat of a microscope lamp affect heart rate?  How do we measure and establish resting heart rate in light of the above?

Pilot studies  A pilot study will be required to resolve this before the protocol is suitable for use in an investigation.  What range of the dependent variable can be used and how can it be delivered to the Daphnia and with what accuracy?  If we are looking at concentrations of the independent variable how can we be sure of what dose actually is being delivered to the Daphnia?

Pilot studies  How many samples should be taken? At least four different values of ethanol concentration should be used and replicated at least twice. This also raises the issue of sample size, i.e. how many Daphnia should be used per concentration per replicate to be valid? As many as you can manage within your time constraints but a minimum of five per concentration/replicate should be attempted. An average of two Daphnia is still a poor sample.

Success Criteria  Explain the need for a pilot study in terms of the dependent variable, the independent variable, confounding variables and replicates.