KA3.2b - Experimental Design Flashcards
KA3.2 - Experimentation (31 cards)
Define ‘independent variable’
the variable that is changed in a scientific experiment by the investigator
State the two types of independent variable?
- Continous (e.g. temperature)
- Discrete (e.g. different inhibitors of an enzyme)
Define ‘dependent variable’
the variable being measured in a scientific experiment.
State the two types of dependent variable?
- Continous (e.g. absorbance of a solution)
- Discrete (e.g. type of behaviour existed by an organism following a stimulus)
Define ‘control group’
a group in the experiment which a variable is not being tested
e.g. in a drug trial they would not get the drug being tested
Why should a control group always be included in investigations?
for comparison to the treatment group
e.g. when investigating the effect of caffeine on Daphnia heart rate, a suitable control group would involve measuring Daphnia heart rate in the absence of caffeine.
State the two types of experimental design
- Simple
- Multifactorial
Define ‘simple experiment’
involves measuring the effect of one independent variable on a dependent variable.
e.g. measuring the effect of temperature on enzyme activity.
Define ‘multifactorial experiment’
involves a combination of more than one independent variable or combination of treatments.
e.g. investigating the effect of multiple drugs simultaneously
What is an advantage of a simple experiment?
Easier to conduct and easier to control with respect to laboratory conditions, which is important for drawing valid conclusions.
What is a drawback of simple experiment?
findings may not be applicable to a wider setting
Define ‘observational study’
where the investigator uses groups that already exist, so there is no truly independent variable.
What are observational studies good at detecting, and what are they less useful for?
Good = detecting correlation (an association or relationship between two variables)
Bad = determining causation because they do not directly test a hypothesis
Define ‘correlation’
describes the relationship between variables
Define ‘causation’
the process of causing something to happen
When can causation exist?
If the changes in the value of the independent variable are known to directly cause changes to the value of the dependent variable.
Define ‘posititive correlation’
An increase in one variable is accompanied by an increase in the other variable.
Define ‘negative correlation’
an increase in one variable is accompanied by a decrease in the other variable
How is the strength of correlation determined?
proportional to the spread of values from the line of best fit.
e.g. the tighter the dots are the stronger the correlation
What are scatter diagrams used for?
Investigate the relationship between two variables where there is no independent variable.
Define ‘confounding variable’
other variables, besides the independent variable, that may affect the dependent variable
e.g. temperature of the enzyme
How should confounding variables be managed?
They 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 is a randomised block design used for?
can be used in cases where confounding variables cannot easily be controlled.
Define ‘randomised block design’
Distributing randomised blocks of treatment and control groups so that the influence of any confounding variable is likely to be the same across the groups.
